<?php

error_reporting(-1);
mb_internal_encoding('utf-8');

$input = "-7/4*15+8-5=";

$res     = 0;
$op      = "";
$numbers = preg_split("/(\\s*[\\*\\-\\+\\/\\=]\\s*)/", $input, -1, PREG_SPLIT_NO_EMPTY | PREG_SPLIT_DELIM_CAPTURE);
//print_r($numbers);

foreach ($numbers as $key => $val) {
    
    $numbers[$key] = trim($val);
    
    if (preg_match("/[\\+\\-\\/\\*]/", $val)) {
        $op = $val;
        continue;
    } elseif (is_numeric($val)) {
        switch ($op) {
            case "+":
            case "":
                $res += $val;
                break;
            case "-":
                $res -= $val;
                break;
            case "*":
                $res *= $val;
                break;
            case "/":
                $res /= $val;
                break;
            default:
                echo "Wrong input!";
        }
        continue;
    } elseif ($val == "=") {
        $res = round($res, 2);
        echo "$res \n";
        exit;
    } else
        echo "Wrong input!";
    exit;
}
?>