<?php
$input = '-200 + 1=';
echo "Было: $input...\n";
preg_match_all('/([0-9]+)|(\+|\-|\*|\/|\=)/', $input, $array);
$sum = $array[0][0];
foreach ($array[0] as $key => $value)
  {
    if ($array[0][$key + 1] == '=')
      {
        echo "Стало: $sum";
        break;
      }
    else
      {
        $operand2  = $array[0][$key + 2];
        $operation = $array[0][$key + 1];
        switch ($operation)
        {
            case "+":
                $sum += $operand2;
                break;
            case "-":
                $sum -= $operand2;
                break;
            case "*":
                $sum *= $operand2;
                break;
            case "/":
                $sum /= $operand2;
                break;
        }
      }
  }