In PHP, references allow you to create multiple names for the same variable, enabling you to access and manipulate its value through different identifiers. They are denoted by the ampersand symbol & placed before the variable name during declaration or assignment.
When a reference is used in a foreach loop instead of using a simple variable, the reference remains assigned and keeps its "value"
which is a reference, even after the foreach execution.
Not unsetting the reference can lead to bugs later in the code, as most of the time, this behavior is different from what the developer is expecting. For example, the reference may be used incorrectly with the previous value.
To avoid unexpected side effects, it is recommended to always unset a reference that is used in a foreach loop.
Unset the reference that is used in the foreach loop.
$arr = array(1, 2, 3);
foreach ($arr as &$value) { // Noncompliant; $value is still alive after the loop and references the last item of the array: $arr[2]
$value = $value * 2;
}
$value = 'x';
$arr = array(1, 2, 3);
foreach ($arr as &$value) { // Compliant; there is no risk to use by mistake the content of $value pointing to $arr[2]
$value = $value * 2;
}
unset($value);
$value = 'x';
PHP Documentation: