Description:
------------
This is a feature request / RFC for the following use-case:

$res = [];
foreach ($arr as $i) {
    foreach (make_res($i) as $v) {
        $res[] = $v;
    }
}

Array_merge in loop is very sloop so it is not a solution.

which I propose to shorten to:

$res = [];
foreach ($arr as $i) {
    $res[...] = make_res($i);
}

Appending multiple elements to an array is very common use-case.