Subtract a number from array starting backwards from tail ([1,2,3] - 4 = [1,1,0])

I've prepared a small sample code which should work:

$arr1 = array( 1,2,3,4); //-2
$arr2 = array( 1,2,3,4,0,0); //-3
$arr3 = array( 1,2,3,4,0,); //-6


array_subtract( $arr1, 2 );
array_subtract( $arr2, 3 );
array_subtract( $arr3, 6 );

print_r($arr1);
print_r($arr2);
print_r($arr3);

//heres a handy function which requires the array and the subtraction value as input 
function array_subtract( &$array, $sub ) { 
    $size = sizeof( $array ) - 1;

    for( $i = $size; $i >= 0; $i-- ) :
        if ($sub > 0 ) :
            if ( $array[$i] > $sub ) :
                $array[$i] -= $sub;
                $sub = 0;
            else : 
                $sub -= $array[$i];
                $array[$i] -= $array[$i];
            endif;
        else :
            break;
        endif;
    endfor;    
}

The print_r output looks like

Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 2
)
Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 1
    [4] => 0
    [5] => 0
)
Array
(
    [0] => 1
    [1] => 2
    [2] => 1
    [3] => 0
    [4] => 0
)