Looping the
Loop with
SPL
Iterators

Looping the Loop with SPL
Iterators
What is an Iterator?
An Iterator is an object that enables a programmer to
traverse a container, particularly lists.

Looping the Loop with SPL
Iterators
What is an Iterator?
$dataSet = ['A', 'B', 'C'];
foreach($dataSet as $key => $value) {
echo "{$key} => {$value}", PHP_EOL;
}

Looping the Loop with SPL
Iterators
What is an Iterator?
0 => A
1 => B
2 => C

Looping the Loop with SPL
Iterators
What is an Iterator?
$data = ['A', 'B', 'C'];
$dataSet = new ArrayIterator($data);
foreach($dataSet as $key => $value) {
echo "{$key} => {$value}", PHP_EOL;
}

Looping the Loop with SPL
Iterators
What is an Iterator?
0 => A
1 => B
2 => C

Looping the Loop with SPL
Iterators
What is an Iterator?
interface Iterator extends Traversable {
/* Methods */
abstract public current ( void ) : mixed
abstract public key ( void ) : scalar
abstract public next ( void ) : void
abstract public rewind ( void ) : void
abstract public valid ( void ) : bool
}

Looping the Loop with SPL
Iterators
What is an Iterator?
$data = ['A', 'B', 'C’];
$iterator = new ArrayIterator($data);
$iterator->rewind();
while ($iterator->valid()) {
$key = $iterator->key();
$value = $iterator->current();
echo "{$key} => {$value}", PHP_EOL;
$iterator->next();
}

Looping the Loop with SPL
Iterators
What is an Iterator?
0 => A
1 => B
2 => C

Looping the Loop with SPL
Iterators
The Iterable Tree
iterable
array
Traversable
Iterator
Generator
IteratorAggregate

Looping the Loop with SPL
Iterators
The Iterable Tree
Not all iterables are equal
You can count the elements in an array !
Can you count the elements in an Iterator?

Looping the Loop with SPL
Iterators
The Iterable Tree
Not all iterables are equal
You can access the elements of an array by
their index !
Can you access the elements of an Iterator
by index ?

Looping the Loop with SPL
Iterators
The Iterable Tree
Not all iterables are equal
You can rewind an Iterator !
Can you rewind a Generator ?

Looping the Loop with SPL
Iterators
The Iterable Tree
Not all iterables are equal
Iterator_* functions (e.g. iterator_to_array)
work on Iterators ?
Can you use iterator_* functions on an
IteratorAggregate ?

Looping the Loop with SPL
Iterators

Looping the Loop with SPL
Iterators
class UserRepository {
// ...
public function all(): array {
$userQuery = 'SELECT * FROM `users` ...';
$statement = $this->pdo->prepare($userQuery);
$statement->execute();
$users = $statement->fetchAll();
return array_map(fn(array $user): User => User::fromArray($user), $users);
}
}

Looping the Loop with SPL
Iterators
class UserRepository {
// ...
/**
* @return User[]
*/
public function all(): array {
$userQuery = 'SELECT * FROM `users` ...';
$statement = $this->pdo->prepare($userQuery);
$statement->execute();
$users = $statement->fetchAll();
return array_map(fn(array $user): User => User::fromArray($user), $users);
}
}

Looping the Loop with SPL
Iterators
class UserList extends ArrayIterator {
}

Looping the Loop with SPL
Iterators
ArrayIterator
Implements:
Countable
ArrayAccess
Serializable
SeekableIterator (which entends Iterator)

Looping the Loop with SPL
Iterators
class UserRepository {
// ...
public function all(): UserList {
$userQuery = 'SELECT * FROM `users` ...';
$statement = $this->pdo->prepare($userQuery);
$statement->execute();
$users = $statement->fetchAll();
return new UserList(
array_map(fn(array $user): User => User::fromArray($user), $users)
);
}
}

Looping the Loop with SPL
Iterators
ArrayIterator
Implements:
Countable

Looping the Loop with SPL
Iterators
ArrayIterator – Countable
$userData = ['user1', 'user2', 'user3', 'user4'];
$userList = new UserList($userData);
$userCount = count($userList);
echo "There are {$userCount} users in this list", PHP_EOL;

Looping the Loop with SPL
Iterators
ArrayIterator – Countable
$userData = ['user1', 'user2', 'user3', 'user4'];
$userList = new UserList($userData);
echo "There are {$userList->count()} users in this list", PHP_EOL;

Looping the Loop with SPL
Iterators
ArrayIterator – Countable
There are 4 users in this list

Looping the Loop with SPL
Iterators
ArrayIterator
Implements:
Countable
ArrayAccess

Looping the Loop with SPL
Iterators
ArrayIterator – ArrayAccess
$userData = ['user1', 'user2', 'user3', 'user4'];
$userList = new UserList($userData);
echo "The 3rd entry in the list is {$userList[2]}", PHP_EOL;

Looping the Loop with SPL
Iterators
ArrayIterator – ArrayAccess
The 3rd entry in the list is user3

Looping the Loop with SPL
Iterators
ArrayIterator
Implements:
Countable
ArrayAccess
Serializable

Looping the Loop with SPL
Iterators
ArrayIterator – Serializable
$userData = ['user1', 'user2', 'user3', 'user4'];
$userList = new UserList($userData);
var_dump(serialize($userList));

Looping the Loop with SPL
Iterators
ArrayIterator – Serializable
string(117)
"O:8:"UserList":4:{i:0;i:0;i:1;a:4:{i:0;s:5:"user1";
i:1;s:5:"user2";i:2;s:5:"user3";i:3;s:5:"user4";}i:2;
a:0:{}i:3;N;}"

Looping the Loop with SPL
Iterators
ArrayIterator
Implements:
Countable
ArrayAccess
Serializable
SeekableIterator (which entends Iterator)

Looping the Loop with SPL
Iterators
ArrayIterator – SeekableIterator
$userData = ['user1', 'user2', 'user3', 'user4'];
$userList = new UserList($userData);
$userList->seek(2);
while ($userList->valid()) {
echo $userList->current(), PHP_EOL;
$userList->next();
}

Looping the Loop with SPL
Iterators
ArrayIterator – SeekableIterator
user3
user4

Looping the Loop with SPL
Iterators
ArrayIterator
Implements:
Countable
ArrayAccess
Serializable
SeekableIterator (which entends Iterator)
Also provides methods for sorting
Doesn’t work with array_* functions

Looping the Loop with SPL
Iterators

Looping the Loop with SPL
Iterators

Looping the Loop with SPL
Iterators
LimitIterator
$userData = ['user1', 'user2', 'user3', 'user4'];
$userList = new LimitIterator(
new UserList($userData),
2
);
foreach($userList as $value) {
echo $value, PHP_EOL;
}

Looping the Loop with SPL
Iterators
LimitIterator
user3
user4

Looping the Loop with SPL
Iterators
InfiniteIterator
$weekdayNames = ['Sun', 'Mon', 'Tues', 'Wed', 'Thurs', 'Fri', 'Sat'];
$todayOffset = (int) (new DateTime('2020-09-08'))->format('w');
$workdayList = new LimitIterator(
new InfiniteIterator( new ArrayIterator($weekdayNames) ),
$todayOffset
);
$counter = 0;
foreach($workdayList as $dayOfWeek) {
echo $dayOfWeek, PHP_EOL;
if (++$counter === 14) break;
}

Looping the Loop with SPL
Iterators
InfiniteIterator
Tues
Wed
Thurs
Fri
Sat
Sun
Mon
Tues
Wed
Thurs
Fri
Sat
Sun
Mon

Looping the Loop with SPL
Iterators
CallbackFilterIterator
$weekdayNames = ['Sun', 'Mon', 'Tues', 'Wed', 'Thurs', 'Fri', 'Sat'];
$todayOffset = (int) (new DateTime('2020-09-08'))->format('w');
$workdayList = new LimitIterator(
new CallbackFilterIterator(
new InfiniteIterator( new ArrayIterator($weekdayNames) ),
fn($dayName): bool => $dayName !== 'Sat' && $dayName !== 'Sun'
),
$todayOffset – 1
);
$counter = 0;
foreach($workdayList as $dayOfWeek) {
echo $dayOfWeek, PHP_EOL;
if (++$counter === 10) break;
}

Looping the Loop with SPL
Iterators
CallbackFilterIterator
Tues
Wed
Thurs
Fri
Mon
Tues
Wed
Thurs
Fri
Mon

Looping the Loop with SPL
Iterators

Looping the Loop with SPL
Iterators
class UserRepository {
// ...
public function all(): UserList {
$userQuery = 'SELECT * FROM `users` ...';
$statement = $this->pdo->prepare($userQuery);
$statement->execute();
$users = $statement->fetchAll();
return new UserList(
array_map(fn(array $user): User => User::fromArray($user), $users)
);
}
}

Looping the Loop with SPL
Iterators
class UserRepository {
// ...
public function all(): Generator {
$userQuery = 'SELECT * FROM `users` ...';
$statement = $this->pdo->prepare($userQuery);
$statement->execute();
while ($user = $statement->fetch()) {
yield User::fromArray($user);
}
}
}

Looping the Loop with SPL
Iterators
class UserRepository {
// ...
/**
* @return Generator|User[]
*/
public function all(): Generator {
$userQuery = 'SELECT * FROM `users` ...';
$statement = $this->pdo->prepare($userQuery);
$statement->execute();
while ($user = $statement->fetch()) {
yield User::fromArray($user);
}
}
}

Looping the Loop with SPL
Iterators
class UserList implements IteratorAggregate {
private PDOStatement $pdoStatement;
public function __construct(PDOStatement $pdoStatement) {
$this->pdoStatement = $pdoStatement;
}
public function getIterator(): Traversable {
$statement->execute();
while ($user = $this->pdoStatement->fetch()) {
yield User::fromArray($user);
}
}
}

Looping the Loop with SPL
Iterators
class UserRepository {
// ...
public function all(): UserList {
$userQuery = 'SELECT * FROM `users` ...';
$statement = $this->pdo->prepare($userQuery);
return new UserList($statement);
}
}

Looping the Loop with SPL
Iterators

Looping the Loop with SPL
Iterators
A Functional Guide to Cat Herding with PHP
Generators
https://markbakeruk.net/2016/01/19/a-functional-guide-to-cat-
herding-with-php-generators/

Looping the Loop with SPL
Iterators
Filtering and Mapping with SPL Iterators
https://markbakeruk.net/2020/01/05/filtering-and-mapping-with-
spl-iterators/

Looping the Loop with SPL
Iterators
Parallel Looping in PHP with SPL’s
MultipleIterator
https://markbakeruk.net/2019/12/31/parallel-looping-in-php-with-
spls-multipleiterator/

Looping the Loop with SPL
Iterators
Iterating PHP Iterators
By Cal Evans
https://leanpub.com/iteratingphpiterators

Looping the Loop with SPL
Iterators
Mastering the SPL Library
by Joshua Thijssen
https://www.phparch.com/books/mastering-the-spl-library/

Who am I?
Mark Baker
Senior Software Engineer
MessageBird BV, Amsterdam
Coordinator and Developer of:
Open Source PHPOffice library
PHPExcel, PHPWord, PHPPowerPoint, PHPProject, PHPVisio
Minor contributor to PHP core (SPL Datastructures)
Other small open source libraries available on github
@Mark_Baker
https://github.com/MarkBaker
http://uk.linkedin.com/pub/mark-baker/b/572/171
http://markbakeruk.net