Very Slow Solution to is_summable() func

<?php
function is_summable($numbers, $sum) {
    //Sort provided numbers and assign numerical keys.
    asort($numbers);
    $numbers = array_values($numbers);

    //Var for additions and var for number of provided numbers.
    $total = 0;
    $numbers_length = count($numbers);

    //Empty var to fill below.
    $code = '';

    //Loop and add for() loops.
    for ($i = 0; $i < $numbers_length; $i++) {
        $code .= 'for ($n' . $i . ' = 0; $n' . $i . ' < ' . $numbers_length . '; $n' . $i . '++) {';

        if ($i != 0) {
            $code .= 'if ($n' . $i . ' != $n' . ($i - 1) . ') {';
        }

        $code .= '$total += intval($numbers[$n' . $i . ']);';
        $code .= 'if ($total == $sum) {';
        $code .= 'return true;';
        $code .= '}';
    }

    //Add ending bracket for for() loops above.
    for ($l = 0; $l < $numbers_length; $l++) {
        $code .= '$total -= intval($numbers[$n' . $i . ']);';
        if ($l != 0) {
            $code .= '}';
        }
        $code .= '}';
    }

    //Finally, eval the code.
    eval($code);

    //If "true" not returned above, return false.
    return false;
}

$num_arr = array(1,2,3,4,5,6,7,8,9);
var_dump(is_summable($num_arr, 24));
?>