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.

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 generator function 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 function can simply exit, and the calling code continues just as if an array has run out of values.

Note:

A generator cannot return a value: doing so will result in a compile error. An empty return statement is valid syntax within a generator and it will terminate the generator.

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.

Caution

If you use yield in an expression context (for example, on the right hand side of an assignment), you must surround the yield statement with parentheses in PHP 5. For example, this is valid:

$data = (yield $value);

But this is not, and will result in a parse error in PHP 5:

$data = yield $value;

The parenthetical restrictions do not apply in PHP 7.

This syntax may be used in conjunction with the Generator::send() method.

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
Caution

As with the simple value yields shown earlier, yielding a key/value pair in an expression context requires the yield statement to be parenthesised:

$data = (yield $key => $value);

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(13) 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

In PHP 7, 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.

Example #5 Basic use of yield from

<?php
function count_to_ten() {
    
yield 1;
    
yield 2;
    
yield from [34];
    
yield from new ArrayIterator([56]);
    
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 #6 yield from and return values

<?php
function count_to_ten() {
    
yield 1;
    
yield 2;
    
yield from [34];
    
yield from new ArrayIterator([56]);
    
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
add a note add a note

User Contributed Notes 9 notes

up
50
Adil lhan (adilmedya at gmail dot com)
3 years ago
For example yield keyword with Fibonacci:

function getFibonacci()
{
    $i = 0;
    $k = 1; //first fibonacci value
    yield $k;
    while(true)
    {
        $k = $i + $k;
        $i = $k - $i;
        yield $k;       
    }
}

$y = 0;

foreach(getFibonacci() as $fibonacci)
{
    echo $fibonacci . "\n";
    $y++;   
    if($y > 30)
    {
        break; // infinite loop prevent
    }
}
up
10
info at boukeversteegh dot nl
1 year ago
[This comment replaces my previous comment]

You can use generators to do lazy loading of lists. You only compute the items that are actually used. However, when you want to load more items, how to cache the ones already loaded?

Here is how to do cached lazy loading with a generator:

<?php
class CachedGenerator {
    protected
$cache = [];
    protected
$generator = null;

    public function
__construct($generator) {
       
$this->generator = $generator;
    }

    public function
generator() {
        foreach(
$this->cache as $item) yield $item;

        while(
$this->generator->valid() ) {
           
$this->cache[] = $current = $this->generator->current();
           
$this->generator->next();
            yield
$current;
        }
    }
}
class
Foobar {
    protected
$loader = null;

    protected function
loadItems() {
        foreach(
range(0,10) as $i) {
           
usleep(200000);
            yield
$i;
        }
    }

    public function
getItems() {
       
$this->loader = $this->loader ?: new CachedGenerator($this->loadItems());
        return
$this->loader->generator();
    }
}

$f = new Foobar;

# First
print "First\n";
foreach(
$f->getItems() as $i) {
    print
$i . "\n";
    if(
$i == 5 ) {
        break;
    }
}

# Second (items 1-5 are cached, 6-10 are loaded)
print "Second\n";
foreach(
$f->getItems() as $i) {
    print
$i . "\n";
}

# Third (all items are cached and returned instantly)
print "Third\n";
foreach(
$f->getItems() as $i) {
    print
$i . "\n";
}
?>
up
9
Harun Yasar harunyasar at mail dot com
11 months ago
That is a simple fibonacci generator.

<?php
function fibonacci($item) {
   
$a = 0;
   
$b = 1;
    for (
$i = 0; $i < $item; $i++) {
        yield
$a;
       
$a = $b - $a;
       
$b = $a + $b;
    }
}

# give me the first ten fibonacci numbers
$fibo = fibonacci(10);
foreach (
$fibo as $value) {
    echo
"$value\n";
}
?>
up
3
zilvinas at kuusas dot lt
6 months ago
Do not call generator functions directly, that won't work.

<?php

function my_transform($value) {
   
var_dump($value);
    return
$value * 2;
}

function
my_function(array $values) {
    foreach (
$values as $value) {
        yield
my_transform($value);
    }
}

$data = [1, 5, 10];
// my_transform() won't be called inside my_function()
my_function($data);

# my_transform() will be called.
foreach (my_function($data) as $val) {
   
// ...
}
?>
up
3
christophe dot maymard at gmail dot com
1 year ago
<?php
//Example of class implementing IteratorAggregate using generator

class ValueCollection implements IteratorAggregate
{
    private
$items = array();
   
    public function
addValue($item)
    {
       
$this->items[] = $item;
        return
$this;
    }
   
    public function
getIterator()
    {
        foreach (
$this->items as $item) {
            yield
$item;
        }
    }
}

//Initializes a collection
$collection = new ValueCollection();
$collection
       
->addValue('A string')
        ->
addValue(new stdClass())
        ->
addValue(NULL);

foreach (
$collection as $item) {
   
var_dump($item);
}
up
2
Shumeyko Dmitriy
2 years ago
This is little example of using generators with recursion. Used version of php is 5.5.5
[php]
<?php
define
("DS", DIRECTORY_SEPARATOR);
define ("ZERO_DEPTH", 0);
define ("DEPTHLESS", -1);
define ("OPEN_SUCCESS", True);
define ("END_OF_LIST", False);
define ("CURRENT_DIR", ".");
define ("PARENT_DIR", "..");

function
DirTreeTraversal($DirName, $MaxDepth = DEPTHLESS, $CurrDepth = ZERO_DEPTH)
{
  if ((
$MaxDepth === DEPTHLESS) || ($CurrDepth < $MaxDepth)) {
   
$DirHandle = opendir($DirName);
    if (
$DirHandle !== OPEN_SUCCESS) {
      try{
        while ((
$FileName = readdir($DirHandle)) !== END_OF_LIST) { //read all file in directory
         
if (($FileName != CURRENT_DIR) && ($FileName != PARENT_DIR)) {
           
$FullName = $DirName.$FileName;
            yield
$FullName;
            if(
is_dir($FullName)) { //include sub files and directories
             
$SubTrav = DirTreeTraversal($FullName.DS, $MaxDepth, ($CurrDepth + 1));
              foreach(
$SubTrav as $SubItem) yield $SubItem;
            }
          }
        }
      } finally {
       
closedir($DirHandle);
      }
    }
  }
}

$PathTrav = DirTreeTraversal("C:".DS, 2);
print
"<pre>";
foreach(
$PathTrav as $FileName) printf("%s\n", $FileName);
print
"</pre>";
[/
php]
up
1
Hayley Watson
5 months ago
If for some strange reason you need a generator that doesn't yield anything, an empty function doesn't work; the function needs a yield statement to be recognised as a generator.

<?php

function gndn()
{
}

foreach(
gndn() as $it)
{
    echo
'FNORD';
}

?>

But it's enough to have the yield syntactically present even if it's not reachable:

<?php

function gndn()
{
    if(
false) { yield; }
}

foreach(
gndn() as $it)
{
    echo
'FNORD';
}

?>
up
-2
dejiakala at gmail dot com
1 year ago
Another Fibonacci sequence with yield keyword:

<?php

function getFibonacci($first, $second, $total) {
  yield
$first;
  yield
$second;
  for (
$i = 1, $total -= 2; $i <= $total; $i++) {
   
$sum = $first + $second;
   
$first = $second;
   
$second = $sum;
    yield
$sum;
  }
}

// Generate first 10 numbers of the Fibonacci sequence starting from 0, 1
foreach (getFibonacci(0, 1, 10) as $fibonacci) {
 
// 0 1 1 2 3 5 8 13 21 34
 
echo $fibonacci . " ";
}

?>
up
-17
denshadewillspam at HOTMAIL dot com
2 years ago
Note that you can't use count() on generators.

/**
* @return integer[]
*/
function xrange() {
    for ($a = 0; $a < 10; $a++)
    {
        yield $a;
    }
}

function mycount(Traversable $traversable)
{
    $skip = 0;
    foreach($traversable as $skip)
    {
        $skip++;
    }
    return $skip;
}
echo "Count:" . count(xrange()). PHP_EOL;
echo "Count:" . mycount(xrange()). PHP_EOL;
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