array_push() in function scope not affecting variable in global scope

I have array_push() inside of a function and need to run it inside a foreach() filling the new array. Unfortunately I can't see why this does not work.

$mylist = array('house', 'apple', 'key', 'car');
$mailarray = array();

foreach ($mylist as $key) {
    online($key, $mailarray);
}

function online($thekey, $mailarray) {
    array_push($mailarray, $thekey);

}
print_r($mailarray);

This is a sample function, it has more functionality and that's why I need to maintain the idea.

Solution

PHP treats arrays as a sort of “value type” by default (copy on write). You can pass it by reference:

function online($thekey, &$mailarray) {
    $mailarray[] = $thekey;
}