International PHP Conference Berlin 2025

Generators overview

(PHP 5 >= 5.5.0, PHP 7, PHP 8)

Generators provide an easy way to implement simple iterators without the overhead or complexity of implementing a class that implements the Iterator interface.

A generator offers a convenient way to provide data to foreach loops without having to build an array in memory ahead of time, which may cause the program to exceed a memory limit or require a considerable amount of processing time to generate. Instead, a generator function can be used, which is the same as a normal function, except that instead of returning once, a generator can yield as many times as it needs to in order to provide the values to be iterated over. Like with iterators, random data access is not possible.

A simple example of this is to reimplement the range() function as a generator. The standard range() function has to generate an array with every value in it and return it, which can result in large arrays: for example, calling range(0, 1000000) will result in well over 100 MB of memory being used.

As an alternative, we can implement an xrange() generator, which will only ever need enough memory to create an Iterator object and track the current state of the generator internally, which turns out to be less than 1 kilobyte.

Example #1 Implementing range() as a generator

<?php
function xrange($start, $limit, $step = 1) {
if (
$start <= $limit) {
if (
$step <= 0) {
throw new
LogicException('Step must be positive');
}

for (
$i = $start; $i <= $limit; $i += $step) {
yield
$i;
}
} else {
if (
$step >= 0) {
throw new
LogicException('Step must be negative');
}

for (
$i = $start; $i >= $limit; $i += $step) {
yield
$i;
}
}
}

/*
* Note that both range() and xrange() result in the same
* output below.
*/

echo 'Single digit odd numbers from range(): ';
foreach (
range(1, 9, 2) as $number) {
echo
"$number ";
}
echo
"\n";

echo
'Single digit odd numbers from xrange(): ';
foreach (
xrange(1, 9, 2) as $number) {
echo
"$number ";
}
?>

Il precedente esempio visualizzerà:

Single digit odd numbers from range():  1 3 5 7 9
Single digit odd numbers from xrange(): 1 3 5 7 9

Generator objects

When a generator function is called, a new object of the internal Generator class is returned. This object implements the Iterator interface in much the same way as a forward-only iterator object would, and provides methods that can be called to manipulate the state of the generator, including sending values to and returning values from it.

add a note

User Contributed Notes 6 notes

up
178
bloodjazman at gmail dot com
11 years ago
for the protection from the leaking of resources
see RFC https://wiki.php.net/rfc/generators#closing_a_generator

and use finnaly

sample code

function getLines($file) {
$f = fopen($file, 'r');
try {
while ($line = fgets($f)) {
yield $line;
}
} finally {
fclose($f);
}
}

foreach (getLines("file.txt") as $n => $line) {
if ($n > 5) break;
echo $line;
}
up
47
montoriusz at gmail dot com
8 years ago
Bear in mind that execution of a generator function is postponed until iteration over its result (the Generator object) begins. This might confuse one if the result of a generator is assigned to a variable instead of immediate iteration.

<?php

$some_state
= 'initial';

function
gen() {
global
$some_state;

echo
"gen() execution start\n";
$some_state = "changed";

yield
1;
yield
2;
}

function
peek_state() {
global
$some_state;
echo
"\$some_state = $some_state\n";
}

echo
"calling gen()...\n";
$result = gen();
echo
"gen() was called\n";

peek_state();

echo
"iterating...\n";
foreach (
$result as $val) {
echo
"iteration: $val\n";
peek_state();
}

?>

If you need to perform some action when the function is called and before the result is used, you'll have to wrap your generator in another function.

<?php
/**
* @return Generator
*/
function some_generator() {
global
$some_state;

$some_state = "changed";
return
gen();
}
?>
up
18
chung1905 at gmail dot com
5 years ago
In addition to the note of "montoriusz at gmail dot com": https://www.php.net/manual/en/language.generators.overview.php#119275

"If you need to perform some action when the function is called and before the result is used, you'll have to wrap your generator in another function."
You can use Generator::rewind instead (https://www.php.net/manual/en/generator.rewind.php)

Sample code:
<?php
/** function/generator definition **/

echo "calling gen()...\n";
$result = gen();
$result->rewind();
echo
"gen() was called\n";

/** iteration **/
?>
up
28
info at boukeversteegh dot nl
8 years ago
Here's how to detect loop breaks, and how to handle or cleanup after an interruption.

<?php
function generator()
{
$complete = false;
try {

while ((
$result = some_function())) {
yield
$result;
}
$complete = true;

} finally {
if (!
$complete) {
// cleanup when loop breaks
} else {
// cleanup when loop completes
}
}

// Do something only after loop completes
}
?>
up
19
lubaev
10 years ago
Abstract test.
<?php

$start_time
=microtime(true);
$array = array();
$result = '';
for(
$count=1000000; $count--;)
{
$array[]=$count/2;
}
foreach(
$array as $val)
{
$val += 145.56;
$result .= $val;
}
$end_time=microtime(true);

echo
"time: ", bcsub($end_time, $start_time, 4), "\n";
echo
"memory (byte): ", memory_get_peak_usage(true), "\n";

?>

<?php

$start_time
=microtime(true);
$result = '';
function
it()
{
for(
$count=1000000; $count--;)
{
yield
$count/2;
}
}
foreach(
it() as $val)
{
$val += 145.56;
$result .= $val;
}
$end_time=microtime(true);

echo
"time: ", bcsub($end_time, $start_time, 4), "\n";
echo
"memory (byte): ", memory_get_peak_usage(true), "\n";

?>
Result:
----------------------------------
| time | memory, mb |
----------------------------------
| not gen | 2.1216 | 89.25 |
|---------------------------------
| with gen | 6.1963 | 8.75 |
|---------------------------------
| diff | < 192% | > 90% |
----------------------------------
up
14
dc at libertyskull dot com
10 years ago
Same example, different results:

----------------------------------
| time | memory, mb |
----------------------------------
| not gen | 0.7589 | 146.75 |
|---------------------------------
| with gen | 0.7469 | 8.75 |
|---------------------------------

Time in results varying from 6.5 to 7.8 on both examples.
So no real drawbacks concerning processing speed.
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