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

Note:

In PHP 5, a generator could not return a value: doing so would result in a compile error. An empty return statement was valid syntax within a generator and it would terminate the generator. Since PHP 7.0, 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.

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.

The value that will be assigned to $data is the value passed to Generator::send(), or NULL if Generator::next() is called instead.

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.

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 use_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 from() {
    
yield 1// key 0
    
yield 2// key 1
    
yield 3// key 2
}
function 
gen() {
    
yield 0// key 0
    
yield from from(); // 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 [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 #7 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
96
Adil lhan (adilmedya at gmail dot com)
5 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
23
info at boukeversteegh dot nl
3 years 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
12
Harun Yasar harunyasar at mail dot com
3 years 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
7
christophe dot maymard at gmail dot com
4 years 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
4
Hayley Watson
2 years 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
4
zilvinas at kuusas dot lt
2 years 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
2
Shumeyko Dmitriy
4 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
-23
denshadewillspam at HOTMAIL dot com
4 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;
up
-6
16630011857 at 163 dot com
29 days ago
The man who does not drink and does not eat meat is incredible!

After reading this sutra, I'm going to quit drinking and eat vegan.

    Excerpt from the "Generous Guang Hua Yan ten Evil Products Sutra":

    Kasyapa Bodhisattva White Buddha said: The Buddha, only the Tathagata for me to explain, do not drink, not meat eaters, how many blessings?

    Buddha-Gloucester:
    If someone, like horse cattle and sheep, glass treasures ying Luo, the country city wife, holding with giving, still less than some people can break wine meat, millions not better than one.

    Replacement is a matter, if someone Bechi gold over 3,000 of the world, holding the use of alms, still less than someone can break wine meat, millions less than one.

    The replacement is a matter, if there is the ability to cast gold for hundreds of, holding the use of giving, still less than some people can break wine meat, millions less than one.

    The replacement is a matter, if someone artificial fan Hua Bao lid, all over the 3,000 world, still less than someone can break wine meat, millions.

    The replacement is a matter, if there are man-made large pagoda, eaves eaves, such as rice hemp Bamboo reed, up to Brahma, as someone can break the wine meat, millions less than the first.

    A good man, not a carnivore, an earthly Bodhisattva, is an extraordinary husband.
    Editor's note: The front content can be seen, even if a person who does not learn Buddha, can insist on eating the whole vegetarian, Ford has boundless. Let's all go vegetarian.

All eggs are not edible, there are children also

  Someone asked the people of Xuanhua: "Why can't vegetarians eat eggs?" "

    The venerable Master said, "No Man or woman (a rooster or a hen) can hatch a chicken." The former people do not understand this truth, he said no, and there is no evidence. The man who eats eggs why does he say such a theory? Is that he wants to eat eggs. Eggs, whether or not a rooster will have chicks, will not be born. "Leng Yan Jing" said: "Eggs only want to live." "Above is the words of the master Xuan Hua is absolutely wrong." After the lying of the man in Xuanhua, he burned more than 4,000 relics to show his life not to play half a sentence of Sakyamuni Buddha, the people of Xuanhua, Inguang and Guang Chin all believe that all animals ' sperm eggs cannot be eaten. I hope we know the cause and effect and don't do anything stupid. Don't eat any food that contains eggs. South No Amitabha ~!

  "The Theory of Explicit understanding" cloud: "All eggs are not edible, have a son also".
-"Big is Tibet" 31st volume of page 882.
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