Definitely relevant for PHP 7+

1. You can't change array during iteration

Foreach WILL NOT LOOP through new values added to the array 
<?php
while inside the loop.
$a = [1, 2, 3];
foreach ($a as $k => $v) {
    echo $v;

    if ($v === 2) {
        $v[] = 4;
    }
}
?>
Output: 123
But the original array was modified: [1, 2, 3, 4]

Foreach WILL LOOP through values deleted from the array while inside the loop.
<?php
$a = [1, 2, 3];
foreach ($a as $k => $v) {
    echo $v;

    if ($v === 2) {
        unset($a[2]);
    }
}
?>
Output: 123
But the original array was modified: [1, 2]

2. But If you iterate by reference using foreach ($arr as &$v) then $arr is turned into a reference and you can change it during iteration

Foreach WILL LOOP through new values added to the array while inside the loop.
<?php
$a = [1, 2, 3];
foreach ($v as &$v) {
    echo $v;

    if ($v === 2) {
        $v[] = 4;
    }
}
?>
Output: 1234

Foreach WILL NOT LOOP through values deleted from the array while inside the loop.
<?php
$a = [1, 2, 3];
foreach ($a as $k => &$v) {
    echo $v;

    if ($v === 2) {
        unset($a[2]);
    }
}
?>
Output: 12