<?php

$input = '0.5/2=';
$inputLength = mb_strlen($input);

$number = 0;
$result = 0;
$op = '';

for ($i = 0; $i < $inputLength; $i++) {

    $char = mb_substr($input, $i, 1);
    if ($char == '+' || $char == '-' || $char == '*' || $char == '='  || $char == '/') {
        $number = floatval($number);
        if ($op == '') {
            $result = $number;
        } elseif ($op == '+') {
            $result += $number;
        } elseif ($op == '-') {
            $result -= $number;
        } elseif ($op == '*') {
            $result *= $number;
        } elseif ($op == '/') {
            $result /= $number;
        }
        if ($char == '=') {
            echo $input . $result;
        }
        $op = $char;
        $number = '';
    } elseif (is_numeric($char)) {
        $char = floatval($char);
        if ($number) {
            $number .= $char;
        } else {
            $number = $char;
        }
    } elseif ($char = '.') {
        $number .= '.';
    } else {
        echo "Неверный символ: $char \n";
        exit;
    }
}