(PHP 5 >= 5.5.0, PHP 7, PHP 8)
生成器提供了一种更容易的方法来实现简单的对象迭代，相比较定义类实现 Iterator 接口的方式，性能开销和复杂性大大降低。
生成器提供了方便的方式来向 foreach 循环提供数据，而无需提前在内存中构建数组，这可能会导致程序超出内存限制或需要相当长的处理时间来生成。相反，可以使用生成器函数，与普通的
function 相同，不同之处在于生成器不是 return
一次，而是可以根据需要多次 yield 以提供要迭代的值。与迭代器一样，无法进行随机数据访问。
一个简单的例子就是使用生成器来重新实现 range() 函数。 标准的 range() 函数需要在内存中生成一个数组包含每一个在它范围内的值，然后返回该数组, 结果就是会产生多个很大的数组。 比如，调用 range(0, 1000000) 将导致内存占用超过 100 MB。
做为一种替代方法, 我们可以实现一个 xrange()
生成器, 只需要足够的内存来创建
Iterator 对象并在内部跟踪生成器的当前状态，这样只需要不到1K字节的内存。
示例 #1 将 range() 实现为生成器
<?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;
}
}
}
/*
* 注意下面range()和xrange()输出的结果是一样的。
*/
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 ";
}
?>以上示例会输出：
Single digit odd numbers from range(): 1 3 5 7 9 Single digit odd numbers from xrange(): 1 3 5 7 9
for the protection from the leaking of resources see RFC https://wiki.php.net/rfc/generators#closing_a_generatorand use finnalysample codefunction 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 $line;}Bear in mind that execution of a generator function is postponed until iteration over its result (the Generator object) begins. This might confuse one if the result of a generator is assigned to a variable instead of immediate iteration.<?php$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; $some_state = "changed"; return gen();}?>In addition to the note of "montoriusz at gmail dot com": https://www.php.net/manual/en/language.generators.overview.php#119275"If you need to perform some action when the function is called and before the result is used, you'll have to wrap your generator in another function."You can use Generator::rewind instead (https://www.php.net/manual/en/generator.rewind.php)Sample code:<?php/** function/generator definition **/echo "calling gen()...\n";$result = gen();$result->rewind();echo "gen() was called\n";/** iteration **/?>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 }?>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% |----------------------------------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.