The new Abstract Syntax Tree (AST) in PHP 7 means the way our PHP code is being executed has changed. Understanding this new fundamental compilation step is key to understanding how our code is being run. To demonstrate, James will show how a basic compiler works and how introducing an AST simplifies this process. We’ll look into how these magical time-warp techniques* can also be used in your code to introspect, analyse and modify code in a way that was never possible before. After seeing this talk, you’ll have a great insight as to the wonders of an AST, and how it can be applied to both compilers and userland code. (*actual magic or time-warp not guaranteed)

1. @asgrim
Climbing the
Abstract Syntax Tree
James Titcumb
ScotlandPHP 2018

2. $ whoami
James Titcumb
www.jamestitcumb.com
www.roave.com
@asgrim

3. @asgrim
How PHP works
PHP code
OpCache
Execute (VM)
Lexer + Parser
Compiler

4. @asgrim
The PHP Lexer
zend_language_scanner.l

5. @asgrim
zend_language_scanner.l
<ST_IN_SCRIPTING>"exit" {
RETURN_TOKEN(T_EXIT);
}
<ST_IN_SCRIPTING>"die" {
RETURN_TOKEN(T_EXIT);
}
<ST_IN_SCRIPTING>"function" {
RETURN_TOKEN(T_FUNCTION);
}

6. @asgrim
zend_language_scanner.l
<ST_IN_SCRIPTING>"exit" {
RETURN_TOKEN(T_EXIT);
}
<ST_IN_SCRIPTING>"die" {
RETURN_TOKEN(T_EXIT);
}
<ST_IN_SCRIPTING>"function" {
RETURN_TOKEN(T_FUNCTION);
}

7. @asgrim
zend_language_scanner.l
<ST_IN_SCRIPTING>"exit" {
RETURN_TOKEN(T_EXIT);
}
<ST_IN_SCRIPTING>"die" {
RETURN_TOKEN(T_EXIT);
}
<ST_IN_SCRIPTING>"function" {
RETURN_TOKEN(T_FUNCTION);
}

8. @asgrim
zend_language_scanner.l
<ST_IN_SCRIPTING>"exit" {
RETURN_TOKEN(T_EXIT);
}
<ST_IN_SCRIPTING>"die" {
RETURN_TOKEN(T_EXIT);
}
<ST_IN_SCRIPTING>"function" {
RETURN_TOKEN(T_FUNCTION);
}

9. @asgrim
zend_language_scanner.l
<ST_DOUBLE_QUOTES,ST_BACKQUOTE,ST_HEREDOC>"${" {
yy_push_state(ST_LOOKING_FOR_VARNAME);
RETURN_TOKEN(T_DOLLAR_OPEN_CURLY_BRACES);
}
<ST_LOOKING_FOR_VARNAME>{LABEL}[[}] {
yyless(yyleng - 1);
zend_copy_value(zendlval, yytext, yyleng);
yy_pop_state();
yy_push_state(ST_IN_SCRIPTING);
RETURN_TOKEN(T_STRING_VARNAME);
}

10. @asgrim
zend_language_scanner.l
<ST_DOUBLE_QUOTES,ST_BACKQUOTE,ST_HEREDOC>"${" {
yy_push_state(ST_LOOKING_FOR_VARNAME);
RETURN_TOKEN(T_DOLLAR_OPEN_CURLY_BRACES);
}
<ST_LOOKING_FOR_VARNAME>{LABEL}[[}] {
yyless(yyleng - 1);
zend_copy_value(zendlval, yytext, yyleng);
yy_pop_state();
yy_push_state(ST_IN_SCRIPTING);
RETURN_TOKEN(T_STRING_VARNAME);
}

11. @asgrim
zend_language_scanner.l
<ST_DOUBLE_QUOTES,ST_BACKQUOTE,ST_HEREDOC>"${" {
yy_push_state(ST_LOOKING_FOR_VARNAME);
RETURN_TOKEN(T_DOLLAR_OPEN_CURLY_BRACES);
}
<ST_LOOKING_FOR_VARNAME>{LABEL}[[}] {
yyless(yyleng - 1);
zend_copy_value(zendlval, yytext, yyleng);
yy_pop_state();
yy_push_state(ST_IN_SCRIPTING);
RETURN_TOKEN(T_STRING_VARNAME);
}

12. @asgrim
zend_language_scanner.l
<ST_DOUBLE_QUOTES,ST_BACKQUOTE,ST_HEREDOC>"${" {
yy_push_state(ST_LOOKING_FOR_VARNAME);
RETURN_TOKEN(T_DOLLAR_OPEN_CURLY_BRACES);
}
<ST_LOOKING_FOR_VARNAME>{LABEL}[[}] {
yyless(yyleng - 1);
zend_copy_value(zendlval, yytext, yyleng);
yy_pop_state();
yy_push_state(ST_IN_SCRIPTING);
RETURN_TOKEN(T_STRING_VARNAME);
}

13. @asgrim
zend_language_scanner.l
<ST_DOUBLE_QUOTES,ST_BACKQUOTE,ST_HEREDOC>"${" {
yy_push_state(ST_LOOKING_FOR_VARNAME);
RETURN_TOKEN(T_DOLLAR_OPEN_CURLY_BRACES);
}
<ST_LOOKING_FOR_VARNAME>{LABEL}[[}] {
yyless(yyleng - 1);
zend_copy_value(zendlval, yytext, yyleng);
yy_pop_state();
yy_push_state(ST_IN_SCRIPTING);
RETURN_TOKEN(T_STRING_VARNAME);
}

14. @asgrim
zend_language_scanner.l
<ST_DOUBLE_QUOTES,ST_BACKQUOTE,ST_HEREDOC>"${" {
yy_push_state(ST_LOOKING_FOR_VARNAME);
RETURN_TOKEN(T_DOLLAR_OPEN_CURLY_BRACES);
}
<ST_LOOKING_FOR_VARNAME>{LABEL}[[}] {
yyless(yyleng - 1);
zend_copy_value(zendlval, yytext, yyleng);
yy_pop_state();
yy_push_state(ST_IN_SCRIPTING);
RETURN_TOKEN(T_STRING_VARNAME);
}

15. @asgrim
zend_language_scanner.l
<ST_DOUBLE_QUOTES,ST_BACKQUOTE,ST_HEREDOC>"${" {
yy_push_state(ST_LOOKING_FOR_VARNAME);
RETURN_TOKEN(T_DOLLAR_OPEN_CURLY_BRACES);
}
<ST_LOOKING_FOR_VARNAME>{LABEL}[[}] {
yyless(yyleng - 1);
zend_copy_value(zendlval, yytext, yyleng);
yy_pop_state();
yy_push_state(ST_IN_SCRIPTING);
RETURN_TOKEN(T_STRING_VARNAME);
}

16. @asgrim
zend_language_scanner.l
<ST_DOUBLE_QUOTES,ST_BACKQUOTE,ST_HEREDOC>"${" {
yy_push_state(ST_LOOKING_FOR_VARNAME);
RETURN_TOKEN(T_DOLLAR_OPEN_CURLY_BRACES);
}
<ST_LOOKING_FOR_VARNAME>{LABEL}[[}] {
yyless(yyleng - 1);
zend_copy_value(zendlval, yytext, yyleng);
yy_pop_state();
yy_push_state(ST_IN_SCRIPTING);
RETURN_TOKEN(T_STRING_VARNAME);
}

17. @asgrim
zend_language_scanner.l
<ST_DOUBLE_QUOTES,ST_BACKQUOTE,ST_HEREDOC>"${" {
yy_push_state(ST_LOOKING_FOR_VARNAME);
RETURN_TOKEN(T_DOLLAR_OPEN_CURLY_BRACES);
}
<ST_LOOKING_FOR_VARNAME>{LABEL}[[}] {
yyless(yyleng - 1);
zend_copy_value(zendlval, yytext, yyleng);
yy_pop_state();
yy_push_state(ST_IN_SCRIPTING);
RETURN_TOKEN(T_STRING_VARNAME);
}

18. @asgrim
The PHP Lexer
zend_language_scanner.l

19. @asgrim
The PHP Lexer
zend_language_scanner.l
re2c

20. @asgrim
The PHP Lexer
zend_language_scanner.l
re2c
zend_language_scanner.c

21. @asgrim
The PHP Parser
zend_language_parser.y

22. @asgrim
zend_language_parser.y
if_stmt:
if_stmt_without_else %prec T_NOELSE { $$ = $1; }
| if_stmt_without_else T_ELSE statement
{ $$ = zend_ast_list_add($1, zend_ast_create(ZEND_AST_IF_ELEM, NULL, $3)); }
;
if_stmt_without_else:
T_IF '(' expr ')' statement
{ $$ = zend_ast_create_list(1, ZEND_AST_IF,
zend_ast_create(ZEND_AST_IF_ELEM, $3, $5)); }
| if_stmt_without_else T_ELSEIF '(' expr ')' statement
{ $$ = zend_ast_list_add($1,
zend_ast_create(ZEND_AST_IF_ELEM, $4, $6)); }
;

23. @asgrim
if_stmt:
if_stmt_without_else %prec T_NOELSE { $$ = $1; }
| if_stmt_without_else T_ELSE statement
{ $$ = zend_ast_list_add($1, zend_ast_create(ZEND_AST_IF_ELEM, NULL, $3)); }
;
if_stmt_without_else:
T_IF '(' expr ')' statement
{ $$ = zend_ast_create_list(1, ZEND_AST_IF,
zend_ast_create(ZEND_AST_IF_ELEM, $3, $5)); }
| if_stmt_without_else T_ELSEIF '(' expr ')' statement
{ $$ = zend_ast_list_add($1,
zend_ast_create(ZEND_AST_IF_ELEM, $4, $6)); }
;
zend_language_parser.y

24. @asgrim
if_stmt:
if_stmt_without_else %prec T_NOELSE { $$ = $1; }
| if_stmt_without_else T_ELSE statement
{ $$ = zend_ast_list_add($1, zend_ast_create(ZEND_AST_IF_ELEM, NULL, $3)); }
;
if_stmt_without_else:
T_IF '(' expr ')' statement
{ $$ = zend_ast_create_list(1, ZEND_AST_IF,
zend_ast_create(ZEND_AST_IF_ELEM, $3, $5)); }
| if_stmt_without_else T_ELSEIF '(' expr ')' statement
{ $$ = zend_ast_list_add($1,
zend_ast_create(ZEND_AST_IF_ELEM, $4, $6)); }
;
zend_language_parser.y

25. @asgrim
if_stmt:
if_stmt_without_else %prec T_NOELSE { $$ = $1; }
| if_stmt_without_else T_ELSE statement
{ $$ = zend_ast_list_add($1, zend_ast_create(ZEND_AST_IF_ELEM, NULL, $3)); }
;
if_stmt_without_else:
T_IF '(' expr ')' statement
{ $$ = zend_ast_create_list(1, ZEND_AST_IF,
zend_ast_create(ZEND_AST_IF_ELEM, $3, $5)); }
| if_stmt_without_else T_ELSEIF '(' expr ')' statement
{ $$ = zend_ast_list_add($1,
zend_ast_create(ZEND_AST_IF_ELEM, $4, $6)); }
;
zend_language_parser.y

26. @asgrim
if_stmt:
if_stmt_without_else %prec T_NOELSE { $$ = $1; }
| if_stmt_without_else T_ELSE statement
{ $$ = zend_ast_list_add($1, zend_ast_create(ZEND_AST_IF_ELEM, NULL, $3)); }
;
if_stmt_without_else:
T_IF '(' expr ')' statement
{ $$ = zend_ast_create_list(1, ZEND_AST_IF,
zend_ast_create(ZEND_AST_IF_ELEM, $3, $5)); }
| if_stmt_without_else T_ELSEIF '(' expr ')' statement
{ $$ = zend_ast_list_add($1,
zend_ast_create(ZEND_AST_IF_ELEM, $4, $6)); }
;
zend_language_parser.y

27. @asgrim
if_stmt:
if_stmt_without_else %prec T_NOELSE { $$ = $1; }
| if_stmt_without_else T_ELSE statement
{ $$ = zend_ast_list_add($1, zend_ast_create(ZEND_AST_IF_ELEM, NULL, $3)); }
;
if_stmt_without_else:
T_IF '(' expr ')' statement
{ $$ = zend_ast_create_list(1, ZEND_AST_IF,
zend_ast_create(ZEND_AST_IF_ELEM, $3, $5)); }
| if_stmt_without_else T_ELSEIF '(' expr ')' statement
{ $$ = zend_ast_list_add($1,
zend_ast_create(ZEND_AST_IF_ELEM, $4, $6)); }
;
zend_language_parser.y

28. @asgrim
if_stmt:
if_stmt_without_else %prec T_NOELSE { $$ = $1; }
| if_stmt_without_else T_ELSE statement
{ $$ = zend_ast_list_add($1, zend_ast_create(ZEND_AST_IF_ELEM, NULL, $3)); }
;
if_stmt_without_else:
T_IF '(' expr ')' statement
{ $$ = zend_ast_create_list(1, ZEND_AST_IF,
zend_ast_create(ZEND_AST_IF_ELEM, $3, $5)); }
| if_stmt_without_else T_ELSEIF '(' expr ')' statement
{ $$ = zend_ast_list_add($1,
zend_ast_create(ZEND_AST_IF_ELEM, $4, $6)); }
;
zend_language_parser.y

29. @asgrim
if_stmt:
if_stmt_without_else %prec T_NOELSE { $$ = $1; }
| if_stmt_without_else T_ELSE statement
{ $$ = zend_ast_list_add($1, zend_ast_create(ZEND_AST_IF_ELEM, NULL, $3)); }
;
if_stmt_without_else:
T_IF '(' expr ')' statement
{ $$ = zend_ast_create_list(1, ZEND_AST_IF,
zend_ast_create(ZEND_AST_IF_ELEM, $3, $5)); }
| if_stmt_without_else T_ELSEIF '(' expr ')' statement
{ $$ = zend_ast_list_add($1,
zend_ast_create(ZEND_AST_IF_ELEM, $4, $6)); }
;
zend_language_parser.y

30. @asgrim
if ($a == 1)
{
a();
}
else if ($b == 1)
{
b();
}
else
{
c();
}
Using the rules to parse

31. @asgrim
if ($a == 1)
{
a();
}
else if ($b == 1)
{
b();
}
else
{
c();
}
Using the rules to parse
if_stmt_without_else (A)

32. @asgrim
if ($a == 1)
{
a();
}
else if ($b == 1)
{
b();
}
else
{
c();
}
Using the rules to parse
if_stmt_without_else (A)
if_stmt_without_else (B)

33. @asgrim
if ($a == 1)
{
a();
}
else if ($b == 1)
{
b();
}
else
{
c();
}
Using the rules to parse
if_stmt_without_else (A)
if_stmt_without_else (B)
if_stmt

34. @asgrim
Zend_language_parser.y (PHP 7.0.10)
if_stmt:
if_stmt_without_else %prec T_NOELSE { $$ = $1; }
| if_stmt_without_else T_ELSE statement
{ $$ = zend_ast_list_add($1, zend_ast_create(ZEND_AST_IF_ELEM, NULL, $3)); }
;
if_stmt_without_else:
T_IF '(' expr ')' statement
{ $$ = zend_ast_create_list(1, ZEND_AST_IF,
zend_ast_create(ZEND_AST_IF_ELEM, $3, $5)); }
| if_stmt_without_else T_ELSEIF '(' expr ')' statement
{ $$ = zend_ast_list_add($1,
zend_ast_create(ZEND_AST_IF_ELEM, $4, $6)); }
;

35. @asgrim
zend_language_parser.y (PHP 5.6.26)
T_IF parenthesis_expr { zend_do_if_cond(&$2, &$1 TSRMLS_CC); }
statement { zend_do_if_after_statement(&$1, 1 TSRMLS_CC); }
void zend_do_if_cond(const znode *cond, znode *closing_bracket_token TSRMLS_DC)
{
int if_cond_op_number = get_next_op_number(CG(active_op_array));
zend_op *opline = get_next_op(CG(active_op_array) TSRMLS_CC);
opline->opcode = ZEND_JMPZ;
SET_NODE(opline->op1, cond);
closing_bracket_token->u.op.opline_num = if_cond_op_number;
SET_UNUSED(opline->op2);
INC_BPC(CG(active_op_array));
}

36. @asgrim
AST is new in PHP 7+

37. @asgrim
How PHP works
PHP code
OpCache
Execute (VM)
Lexer + Parser
Compiler

38. @asgrim
Let’s simplify!

39. @asgrim
First… WTF is AST?

40. @asgrim
AST is just a data structure

41. @asgrim
PHP code
<?php
echo "Hello world";

42. @asgrim
An AST representation
Echo statement
`-- String, value "Hello world"

43. @asgrim
PHP code
<?php
echo "Hello " . "world";

44. @asgrim
An AST representation
Echo statement
`-- Concat
|-- Left
| `-- String, value "Hello "
`-- Right
`-- String, value "world"

45. @asgrim
PHP code
<?php
$a = 5;
$b = 3;
echo $a + ($b * 2);

46. @asgrim
An AST representation
Assign statement
|-- Variable $a
`-- Integer, value 5
Assign statement
|-- Variable $b
`-- Integer, value 3
Echo statement
`-- Add operation
|-- Left
| `-- Variable $a
`-- Right
`-- Multiply operation
|-- Left
| `-- Variable $b
`-- Right
`-- Integer, value 2

47. @asgrim
Why?

48. @asgrim
Faster!*

49. @asgrim
AST compilation
Statements
EchoAssign
Scalar
value: (int)5
Variable
name: $a
Assign
Scalar
value: (int)3
Variable
name: $b
Add op
Right operandLeft operand
Variable
name: $a
Multiply op
Right operandLeft operand
Variable
name: $b
Scalar
value: (int)2

50. @asgrim
AST compilation: pre-order traversal
Statements
EchoAssign
Scalar
value: (int)5
Variable
name: $a
Assign
Scalar
value: (int)3
Variable
name: $b
Add op
Right operandLeft operand
Variable
name: $a
Multiply op
Right operandLeft operand
Variable
name: $b
Scalar
value: (int)2

51. @asgrim
Pre-order traversal: Polish notation
Assign(Variable $a, Scalar 5)
Assign(Variable $b, Scalar 3)
Echo (
Add(
Variable $a,
Multiply( $b, 2 )
)
)

52. @asgrim
Order of precedence
1 + 2 * 3
= 1 + (2 * 3) = 7?
= (1 + 2) * 3 = 9?

53. @asgrim
Order of precedence
1 + 2 * 3
= 1 + (2 * 3) = 7?
= (1 + 2) * 3 = 9?
+ 1 * 2 3

54. @asgrim
Order of precedence
1 + 2 * 3
= 1 + (2 * 3) = 7?
= (1 + 2) * 3 = 9?
+ 1 * 2 3
Operator Left operand
Right operand

55. @asgrim
Order of precedence
1 + 2 * 3
= 1 + (2 * 3) = 7?
= (1 + 2) * 3 = 9?
+ 1 * 2 3
Operator Left operand Right operand
Operator Left operand
Right operand

56. @asgrim
Reverse Polish Notation
1 2 3 * +

57. @asgrim
Reverse Polish Notation
1 2 3 * + The stack

58. @asgrim
Reverse Polish Notation
1 2 3 * + The stack
1

59. @asgrim
Reverse Polish Notation
1 2 3 * + The stack
1
2

60. @asgrim
Reverse Polish Notation
1 2 3 * + The stack
1
2
3

61. @asgrim
Reverse Polish Notation
1 2 3 * + The stack
1
2
3

62. @asgrim
Reverse Polish Notation
1 2 3 * + The stack
1
2
3

63. @asgrim
Reverse Polish Notation
1 2 3 * + The stack
1
6

64. @asgrim
Reverse Polish Notation
1 2 3 * + The stack
1
6

65. @asgrim
Reverse Polish Notation
1 2 3 * + The stack
7

66. @asgrim
Let’s write a compiler (!!!)
In three easy steps…

67. @asgrim
Warning: do not use in production

68. @asgrim
View > Source
https://github.com/asgrim/basic-maths-compiler

69. @asgrim
Define the language
Tokens
● T_ADD (+)
● T_SUBTRACT (-)
● T_MULTIPLY (/)
● T_DIVIDE (*)
● T_INTEGER (d)
● T_WHITESPACE (s+)

70. @asgrim
Step 1: Writing a simple lexer

71. @asgrim
Using regular expressions
private static $matches = [
'/^(+)/' => Token::T_ADD,
'/^(-)/' => Token::T_SUBTRACT,
'/^(*)/' => Token::T_MULTIPLY,
'/^(/)/' => Token::T_DIVIDE,
'/^(d+)/' => Token::T_INTEGER,
'/^(s+)/' => Token::T_WHITESPACE,
];

72. @asgrim
Step through the input string
public function __invoke(string $input) : array
{
$tokens = [];
$offset = 0;
while ($offset < strlen($input)) {
$focus = substr($input, $offset);
$result = $this->match($focus);
$tokens[] = $result;
$offset += strlen($result->getLexeme());
}
return $tokens;
}

73. @asgrim
The matching method
private function match(string $input) : Token
{
foreach (self::$matches as $pattern => $token) {
if (preg_match($pattern, $input, $matches)) {
return new Token($token, $matches[1]);
}
}
throw new RuntimeException(sprintf(
'Unmatched token, next 15 chars were: %s', substr($input, 0, 15)
));
}

74. @asgrim
Step 2: Parsing the tokens

75. @asgrim
Order tokens by operator precedence
/**
* Higher number is higher precedence.
* @var int[]
*/
private static $operatorPrecedence = [
Token::T_SUBTRACT => 0,
Token::T_ADD => 1,
Token::T_DIVIDE => 2,
Token::T_MULTIPLY => 3,
];

76. @asgrim
Order tokens by operator precedence
/** @var Token[] $stack */
$stack = [];
/** @var Token[] $operators */
$operators = [];
while (false !== ($token = current($tokens))) {
if ($token->isOperator()) {
// ...
}
$stack[] = $token;
next($tokens);
}

77. @asgrim
Order tokens by operator precedence
/** @var Token[] $stack */
$stack = [];
/** @var Token[] $operators */
$operators = [];
while (false !== ($token = current($tokens))) {
if ($token->isOperator()) {
// ...
}
$stack[] = $token;
next($tokens);
}

78. @asgrim
Order tokens by operator precedence
/** @var Token[] $stack */
$stack = [];
/** @var Token[] $operators */
$operators = [];
while (false !== ($token = current($tokens))) {
if ($token->isOperator()) {
// ...
}
$stack[] = $token;
next($tokens);
}

79. @asgrim
Order tokens by operator precedence
/** @var Token[] $stack */
$stack = [];
/** @var Token[] $operators */
$operators = [];
while (false !== ($token = current($tokens))) {
if ($token->isOperator()) {
// ...
}
$stack[] = $token;
next($tokens);
}

80. @asgrim
Order tokens by operator precedence
if ($token->isOperator()) {
$tokenPrecedence = self::$operatorPrecedence[$token->getToken()];
while (
count($operators)
&& self::$operatorPrecedence[$operators[count($operators) - 1]->getToken()]
> $tokenPrecedence
) {
$higherOp = array_pop($operators);
$stack[] = $higherOp;
}
$operators[] = $token;
next($tokens);
continue;
}

81. @asgrim
Order tokens by operator precedence
if ($token->isOperator()) {
$tokenPrecedence = self::$operatorPrecedence[$token->getToken()];
while (
count($operators)
&& self::$operatorPrecedence[$operators[count($operators) - 1]->getToken()]
> $tokenPrecedence
) {
$higherOp = array_pop($operators);
$stack[] = $higherOp;
}
$operators[] = $token;
next($tokens);
continue;
}

82. @asgrim
Order tokens by operator precedence
if ($token->isOperator()) {
$tokenPrecedence = self::$operatorPrecedence[$token->getToken()];
while (
count($operators)
&& self::$operatorPrecedence[$operators[count($operators) - 1]->getToken()]
> $tokenPrecedence
) {
$higherOp = array_pop($operators);
$stack[] = $higherOp;
}
$operators[] = $token;
next($tokens);
continue;
}

83. @asgrim
Order tokens by operator precedence
if ($token->isOperator()) {
$tokenPrecedence = self::$operatorPrecedence[$token->getToken()];
while (
count($operators)
&& self::$operatorPrecedence[$operators[count($operators) - 1]->getToken()]
> $tokenPrecedence
) {
$higherOp = array_pop($operators);
$stack[] = $higherOp;
}
$operators[] = $token;
next($tokens);
continue;
}

84. @asgrim
Order tokens by operator precedence
// Clean up by moving any remaining operators onto the token stack
while (count($operators)) {
$stack[] = array_pop($operators);
}
return $stack;

85. @asgrim
Order tokens by operator precedence
1 + 2 * 3
Output stack
Operator stack

86. @asgrim
Order tokens by operator precedence
1 + 2 * 3
1Output stack
Operator stack

87. @asgrim
Order tokens by operator precedence
1 + 2 * 3
1
+
Output stack
Operator stack

88. @asgrim
Order tokens by operator precedence
1 + 2 * 3
1 2
+
Output stack
Operator stack

89. @asgrim
Order tokens by operator precedence
1 + 2 * 3
1 2
+ *
Output stack
Operator stack

90. @asgrim
Order tokens by operator precedence
1 + 2 * 3
1 2 3
+ *
Output stack
Operator stack

91. @asgrim
Order tokens by operator precedence
1 + 2 * 3
1 2 3 *
+ *
Output stack
Operator stack

92. @asgrim
Order tokens by operator precedence
1 + 2 * 3
1 2 3 * +
+
Output stack
Operator stack

93. @asgrim
Create AST
while ($ip < count($tokenStack)) {
$token = $tokenStack[$ip++];
if ($token->isOperator()) {
// (figure out $nodeType)
$right = array_pop($astStack);
$left = array_pop($astStack);
$astStack[] = new $nodeType($left, $right);
continue;
}
$astStack[] = new NodeScalarIntegerValue((int)$token->getLexeme());
}

94. @asgrim
Create AST
while ($ip < count($tokenStack)) {
$token = $tokenStack[$ip++];
if ($token->isOperator()) {
// (figure out $nodeType)
$right = array_pop($astStack);
$left = array_pop($astStack);
$astStack[] = new $nodeType($left, $right);
continue;
}
$astStack[] = new NodeScalarIntegerValue((int)$token->getLexeme());
}

95. @asgrim
Create AST
while ($ip < count($tokenStack)) {
$token = $tokenStack[$ip++];
if ($token->isOperator()) {
// (figure out $nodeType)
$right = array_pop($astStack);
$left = array_pop($astStack);
$astStack[] = new $nodeType($left, $right);
continue;
}
$astStack[] = new NodeScalarIntegerValue((int)$token->getLexeme());
}

96. @asgrim
Create AST
while ($ip < count($tokenStack)) {
$token = $tokenStack[$ip++];
if ($token->isOperator()) {
// (figure out $nodeType)
$right = array_pop($astStack);
$left = array_pop($astStack);
$astStack[] = new $nodeType($left, $right);
continue;
}
$astStack[] = new NodeScalarIntegerValue((int)$token->getLexeme());
}

97. @asgrim
Create AST
NodeBinaryOpAdd (
NodeScalarIntegerValue(1),
NodeBinaryOpMultiply (
NodeScalarIntegerValue(2),
NodeScalarIntegerValue(3)
)
)

98. @asgrim
Step 3: Executing the AST

99. @asgrim
Compile & execute AST
private function compileNode(NodeInterface $node)
{
if ($node instanceof NodeBinaryOpAbstractBinaryOp) {
return $this->compileBinaryOp($node);
}
if ($node instanceof NodeScalarIntegerValue) {
return $node->getValue();
}
}

100. @asgrim
Compile & execute AST
private function compileBinaryOp(NodeBinaryOpAbstractBinaryOp $node)
{
$left = $this->compileNode($node->getLeft());
$right = $this->compileNode($node->getRight());
switch (get_class($node)) {
case NodeBinaryOpAdd::class:
return $left + $right;
case NodeBinaryOpSubtract::class:
return $left - $right;
case NodeBinaryOpMultiply::class:
return $left * $right;
case NodeBinaryOpDivide::class:
return $left / $right;
}
}

101. @asgrim
What does this mean for me?

102. @asgrim
AST in userland

103. @asgrim
php-ast extension
https://github.com/nikic/php-ast

104. @asgrim
php-ast example usage
<?php
require 'path/to/util.php';
$code = <<<'EOC'
<?php
$var = 42;
EOC;
echo ast_dump(astparse_code($code, $version=35)), "n";
// Output:
AST_STMT_LIST
0: AST_ASSIGN
var: AST_VAR
name: "var"
expr: 42

105. @asgrim
astkit
https://github.com/sgolemon/astkit

106. @asgrim

107. @asgrim
astkit example usage
$if = AstKit::parseString(<<<EOD
if (true) {
echo "This is a triumph.n";
} else {
echo "The cake is a lie.n";
}
EOD
);
$if->execute(); // First run, program is as-seen above
$const = $if->getChild(0)->getChild(0);
// Replace the "true" constant in the condition with false
$const->graft(0, false);
// Can also graft other AstKit nodes, instead of constants
$if->execute(); // Second run now takes the else path

108. @asgrim
PhpParser
https://github.com/nikic/PHP-Parser

109. @asgrim
PHP Parser
<?php
use PhpParserParserFactory;
$parser = (new ParserFactory)
->create(ParserFactory::PREFER_PHP7);
print_r($parser->parse(
file_get_contents('ast-demo-src.php')
));

110. @asgrim
Better Reflection
https://github.com/Roave/BetterReflection

111. @asgrim
Better Reflection workflow
Reflector
Source
Locator
PhpParser
Reflection

112. @asgrim
PHP Reflection
$reflection = new ReflectionClass(
MyExampleClass::class
);
$this->assertSame(
'ExampleClass',
$reflection->getShortName()
);

113. @asgrim
Better Reflection
$reflection = (new BetterReflection())
->classReflector()
->reflect(MyExampleClass::class);
$this->assertSame(
'ExampleClass',
$reflection->getShortName()
);

114. @asgrim
Class BetterReflection
public function sourceLocator() : SourceLocator
{
$astLocator = $this->astLocator();
return $this->sourceLocator
?? $this->sourceLocator = new MemoizingSourceLocator(new AggregateSourceLocator([
new PhpInternalSourceLocator($astLocator),
new EvaledCodeSourceLocator($astLocator),
new AutoloadSourceLocator($astLocator),
]));
}
public function classReflector() : ClassReflector
{
return $this->classReflector
?? $this->classReflector = new ClassReflector($this->sourceLocator());
}

115. @asgrim
Given a class structure...
<?php
class Foo
{
private $bar;
public function thing()
{
}
}

116. @asgrim
… we get the AST!
Class, name Foo
|-- Statements
| |-- Property, name bar
| | |-- Type [private]
| | `-- Attributes [start line: 7, end line: 9]
| `-- Method, name thing
| |-- Type [public]
| |-- Parameters [...]
| |-- Statements [...]
| `-- Attributes [start line: 7, end line: 9]
`-- Attributes [start line: 3, end line: 10]

117. @asgrim
What can I use Better Reflection for?

118. @asgrim
Monkey patching example
class MyClass
{
public function foo()
{
return 5;
}
}

119. @asgrim
Monkey patching example
use RoaveBetterReflectionReflectorClassReflector;
use RoaveBetterReflectionSourceLocatorTypeSingleFileSourceLocator;
use RoaveBetterReflectionUtilAutoloadClassLoader;
use RoaveBetterReflectionUtilAutoloadClassLoaderMethodFileCacheLoader;
$loader = new ClassLoader(FileCacheLoader::defaultFileCacheLoader(__DIR__));
// Create the reflection first (without loading)
$classInfo = (new ClassReflector(
new SingleFileSourceLocator(
__DIR__ . '/MyClass.php',
(new BetterReflection())->astLocator()
)
))->reflect('MyClass');
$loader->addClass($classInfo);

120. @asgrim
Monkey patching example
use RoaveBetterReflectionReflectorClassReflector;
use RoaveBetterReflectionSourceLocatorTypeSingleFileSourceLocator;
use RoaveBetterReflectionUtilAutoloadClassLoader;
use RoaveBetterReflectionUtilAutoloadClassLoaderMethodFileCacheLoader;
$loader = new ClassLoader(FileCacheLoader::defaultFileCacheLoader(__DIR__));
// Create the reflection first (without loading)
$classInfo = (new ClassReflector(
new SingleFileSourceLocator(
__DIR__ . '/MyClass.php',
(new BetterReflection())->astLocator()
)
))->reflect('MyClass');
$loader->addClass($classInfo);

121. @asgrim
Monkey patching example
// Override the body...!
$classInfo->getMethod('foo')->setBodyFromClosure(
function () {
return 4;
}
);
$c = new MyClass();
echo $c->foo() . "n";

122. @asgrim
Monkey patching example
// Override the body...!
$classInfo->getMethod('foo')->setBodyFromClosure(
function () {
return 4;
}
);
$c = new MyClass();
echo $c->foo() . "n"; // returns 4, not 5

123. @asgrim
To summarise
● For PHP engine:
○ AST is an efficient data structure to represent code
○ AST means faster compilation (ignoring opcache)
○ Separation in PHP engine for parser and compiler
○ https://wiki.php.net/rfc/abstract_syntax_tree
● Concepts can be used in userland
○ PHP Parser library - https://github.com/nikic/php-parser
○ Better Reflection - https://github.com/Roave/BetterReflection
■ Reflect on not-yet-loaded files
■ Monkey patching in userland code (!)
○ Static analysis opportunities
■ Better Reflection
■ Exakat static analysis (uses own AST)
■ Phan (uses php-ast extension)

124. Any questions?
James Titcumb
@asgrim