<?php

$numbers = array(5, 19, 45, 6, 7, 21);
$operators = array('*', '+', '-', '*', '+');


function rechne($numbers, $operators) {
    $numbers[0] = array(0, '+', $numbers[0]);
    foreach ($operators as $op) {
        $left = array_shift($numbers);
        $right = array_shift($numbers);
        if (in_array($op, array('+', '-'))) {
            array_unshift($numbers, array($left, $op, $right));
        } else {
            $left[2] = array($left[2], $op, $right);
            array_unshift($numbers, $left);
        }
    }

    return rechne_helper($numbers[0]);
}

function rechne_helper ($struct) {
    if (!is_array($struct)) {
        return $struct;
    }

    $left = rechne_helper($struct[0]);
    $right = rechne_helper($struct[2]);
    switch ($struct[1]) {
        case '+':
            return $left + $right;
        break;
        case'-':
            return $left - $right;
        break;
        case '*':
            return $left * $right;
        break;
    }
}

echo rechne($numbers, $operators);