This document outlines PHP functions including function declaration, arguments, returning values, variable scope, static variables, recursion, and useful built-in functions. Functions are blocks of code that perform tasks and can take arguments. They are declared with the function keyword followed by the name and parameters. Functions can return values and arguments are passed by value by default but can also be passed by reference. Variable scope inside functions refers to the local scope unless specified as global. Static variables retain their value between function calls. Recursion occurs when a function calls itself. Useful built-in functions include function_exists() and get_defined_functions().

1. PHP Functions

2. Outline
• Functions
• Function declaration
• Function Arguments
• Variable number of arguments
• Returning Values
• Variable Scope
• Static variables
• Recursion
• Tricks
• Useful Built-in Functions
• Assignment

3. Functions
• Is a sub program.
• A block of code that is defined under a certain name
( function name ).
• Can take arguments.
• Can return a value.
• Function name has the same naming rules as variables
and constants.
• Function names are not case sensitive.

4. Function declaration
function doSomething($arg_1, $arg_2, /* ..., */ $arg_n)
{
echo “something.n";
return $val;
}
Name Arguments
Body
Return

5. Function Arguments
• Passing by value (default – except for objects) :
function multiply($x, $y)
{
echo $x * $y;
}

6. Function Arguments
• Passing by reference :
$x = 10;
$y = 5;
$result = 0;
multiply( $x, $y , $result );
echo $result; // 50
function multiply($x, $y, &$result)
{
$result = $x * $y;
}

7. Function Arguments
• Default argument values:
function doIt( $name = “PHP” )
{
echo “Hello ” . $name;
}
Can be called :
doIt(); // Hello PHP
doIt( “Guys” ); // Hello Guys
• It is preferable that the arguments with default values
should be the last arguments passed to the function.

8. Function Arguments
• PHP emits a warning if the number of the arguments
passed is not the same the number of arguments in the
function declaration.
• PHP does not throw any warning in case the number is
bigger than the arguments in the function declaration.
function doIt( $name )
{
echo “Hello ” . $name;
}
doIt(); // a warning is emitted
doIt( “Elephant”, “Cow” ); // this is OK

9. Demo

10. Variable number of arguments
• PHP allows you to create a function with unlimited
number of arguments.
function displayAll( )
{
$numargs = func_num_args();
$arg_list = func_get_args();
for ($i = 0; $i < $numargs; $i++) {
echo $arg_list[$i] . “ ”;
}
}
displayAll( “This” , “is”, “a”, “good”, “thing.” );

11. Returning Values
• Functions may return values :
function doIt( $name )
{
return “Hello ” . $name;
}
echo doIt(“PHP”); // Hello PHP

12. Variable Scope
$a = 1; /* global scope */
function test()
{
echo $a; /* reference to local scope variable */
}
test(); // echoes an empty string

13. Variable Scope
The GLOBAL keyword:
$a = 1; /* global scope */
function test()
{
global $a; // can also use the $GLOBALS array
echo $a; /* reference to local scope variable */
}
test(); // 1

14. Static variables
• Static variables are initialized only one time.
function test()
{
static $a = 0;
echo $a;
$a++;
}
test(); //0
test(); // 1
test(); // 2

15. Recursion
• Recursion occurs when something contains, or uses, a
similar version of itself.
• In programming, it happens when a function is having
a call to itself.
• The factorial problem :
• 5! = 5 x 4 x 3 x 2 x 1 = 120

16. Recursion
function factorial($number) {
if ($number == 0) return 1;
return $number * factorial($number - 1);
}
echo factorial(6); // 720

17. Tricks
A function inside an IF statement :
if ($condition) {
function doSomething()
{
echo “Will only be defined if the IF condition is true";
}
// some code
doSomething();
}

18. Tricks
A function inside another function:
function doSomething1()
{
function doSomething2()
{
echo "I can only be called from inside doSomething1
n";
}
// some code
doSomething2();
}

19. Tricks
Variable functions:
function display( $x )
{
echo $x;
}
$name = “display”;
$name( “PHP” ); //PHP

20. Tricks
Anonymous functions:
$greet = function($name)
{
echo “Hello “ . $name;
};
$greet(“PHP”); // Hello PHP

21. Useful Built-in Functions
• boolean function_exists( $name )
• Checks whether there is a function with that
$name.
• void get_defined_functions()
• Gets a list of all names of defined functions in the
script.

22. Assignment
- Write a PHP function that calculates the average of passed
parameters (The number of parameters is dynamic).

23. What's Next?
• Arrays.

24. Questions?