No Dangling References

In a foreach loop, a variable is used for looping through the array. If this variable is made into a reference for on the spot modification purpose, the reference to the last element will survive after the end of the loop. When this last reference is reused later, it will apply to the last element of the array.

<?php
$array = array('a', 'b');
$array2 = array('c', 'd');

foreach ($array as &$a);
foreach ($array2 as $a);

print_r($array);
print_r($array2);
?>

In this example, $array[1] finally get assigned with the value 'd', which is the last value in the second loop. Any assignation to $a would affect $array too.

Array ( [0] => a [1] => d ) Array ( [0] => c [1] => d )

It is recommended to unset the loop variable right after the loop, to avoid reusing it later.

Rule Details

This rule targets code that doesn't unset the loop's reference right after usage.

The following code is considered a warning:

<?php
foreach (array(1, 2, 3, 4) as &$value) {
    $value = $value * 2;
}
$other_value *= 2;
unset($value); // don't wait too long to remove $value
?>

The following pattern is considered legit:

<?php
foreach (array(1, 2, 3, 4) as &$value) {
    $value = $value * 2;
}
unset($value);
?>

Further Reading

• foreach in PHP