Generator syntax

A generator function looks just like a normal function, except that instead of returning a value, a generator yields as many values as it needs to. Any function containing yield is a generator function.

When a generator function is called, it returns an object that can be iterated over. When you iterate over that object (for instance, via a foreach loop), PHP will call the object's iteration methods each time it needs a value, then saves the state of the generator when the generator yields a value so that it can be resumed when the next value is required.

Once there are no more values to be yielded, then the generator can simply return, and the calling code continues just as if an array has run out of values.

Note:
A generator can return values, which can be retrieved using Generator::getReturn().

yield keyword

The heart of a generator function is the yield keyword. In its simplest form, a yield statement looks much like a return statement, except that instead of stopping execution of the function and returning, yield instead provides a value to the code looping over the generator and pauses execution of the generator function.

Example #1 A simple example of yielding values

<?php
function gen_one_to_three() {
    for ($i = 1; $i <= 3; $i++) {
        // Note that $i is preserved between yields.
        yield $i;
    }
}

$generator = gen_one_to_three();
foreach ($generator as $value) {
    echo "$value\n";
}
?>

The above example will output:

1
2
3

Note:
Internally, sequential integer keys will be paired with the yielded values, just as with a non-associative array.

Yielding values with keys

PHP also supports associative arrays, and generators are no different. In addition to yielding simple values, as shown above, you can also yield a key at the same time.

The syntax for yielding a key/value pair is very similar to that used to define an associative array, as shown below.

Example #2 Yielding a key/value pair

<?php
/*
 * The input is semi-colon separated fields, with the first
 * field being an ID to use as a key.
 */

$input = <<<'EOF'
1;PHP;Likes dollar signs
2;Python;Likes whitespace
3;Ruby;Likes blocks
EOF;

function input_parser($input) {
    foreach (explode("\n", $input) as $line) {
        $fields = explode(';', $line);
        $id = array_shift($fields);

        yield $id => $fields;
    }
}

foreach (input_parser($input) as $id => $fields) {
    echo "$id:\n";
    echo "    $fields[0]\n";
    echo "    $fields[1]\n";
}
?>

The above example will output:

1:
    PHP
    Likes dollar signs
2:
    Python
    Likes whitespace
3:
    Ruby
    Likes blocks

Yielding null values

Yield can be called without an argument to yield a null value with an automatic key.

Example #3 Yielding nulls

<?php
function gen_three_nulls() {
    foreach (range(1, 3) as $i) {
        yield;
    }
}

var_dump(iterator_to_array(gen_three_nulls()));
?>

The above example will output:

array(3) {
  [0]=>
  NULL
  [1]=>
  NULL
  [2]=>
  NULL
}

Yielding by reference

Generator functions are able to yield values by reference as well as by value. This is done in the same way as returning references from functions: by prepending an ampersand to the function name.

Example #4 Yielding values by reference

<?php
function &gen_reference() {
    $value = 3;

    while ($value > 0) {
        yield $value;
    }
}

/*
 * Note that we can change $number within the loop, and
 * because the generator is yielding references, $value
 * within gen_reference() changes.
 */
foreach (gen_reference() as &$number) {
    echo (--$number).'... ';
}
?>

The above example will output:

2... 1... 0...

Generator delegation via yield from

Generator delegation allows you to yield values from another generator, Traversable object, or array by using the yield from keyword. The outer generator will then yield all values from the inner generator, object, or array until that is no longer valid, after which execution will continue in the outer generator.

If a generator is used with yield from, the yield from expression will also return any value returned by the inner generator.

Caution

Storing into an array (e.g. with iterator_to_array())

yield from does not reset the keys. It preserves the keys returned by the Traversable object, or array. Thus some values may share a common key with another yield or yield from, which, upon insertion into an array, will overwrite former values with that key.

A common case where this matters is iterator_to_array() returning a keyed array by default, leading to possibly unexpected results. iterator_to_array() has a second parameter preserve_keys which can be set to false to collect all the values while ignoring the keys returned by the Generator.

Example #5 yield from with iterator_to_array()

<?php
function inner() {
    yield 1; // key 0
    yield 2; // key 1
    yield 3; // key 2
}
function gen() {
    yield 0; // key 0
    yield from inner(); // keys 0-2
    yield 4; // key 1
}
// pass false as second parameter to get an array [0, 1, 2, 3, 4]
var_dump(iterator_to_array(gen()));
?>

The above example will output:

array(3) {
  [0]=>
  int(1)
  [1]=>
  int(4)
  [2]=>
  int(3)
}

Example #6 Basic use of yield from

<?php
function count_to_ten() {
    yield 1;
    yield 2;
    yield from [3, 4];
    yield from new ArrayIterator([5, 6]);
    yield from seven_eight();
    yield 9;
    yield 10;
}

function seven_eight() {
    yield 7;
    yield from eight();
}

function eight() {
    yield 8;
}

foreach (count_to_ten() as $num) {
    echo "$num ";
}
?>

The above example will output:

1 2 3 4 5 6 7 8 9 10

Example #7 yield from and return values

<?php
function count_to_ten() {
    yield 1;
    yield 2;
    yield from [3, 4];
    yield from new ArrayIterator([5, 6]);
    yield from seven_eight();
    return yield from nine_ten();
}

function seven_eight() {
    yield 7;
    yield from eight();
}

function eight() {
    yield 8;
}

function nine_ten() {
    yield 9;
    return 10;
}

$gen = count_to_ten();
foreach ($gen as $num) {
    echo "$num ";
}
echo $gen->getReturn();
?>

The above example will output:

1 2 3 4 5 6 7 8 9 10