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;
}

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
    }
?>

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.

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%      |
----------------------------------