calculate math expression from a string using eval

Question

I want to calculate math expression from a string. I have read that the solution to this is to use eval(). But when I try to run the following code:

<?php

$ma ="2+10";
$p = eval($ma);
print $p;

?>

It gives me the following error:

Parse error: syntax error, unexpected $end in C:\xampp\htdocs\eclipseWorkspaceWebDev\MandatoryHandinSite\tester.php(4) : eval()'d code on line 1

Does someone know the solution to this problem.

Answer 1

While I don't suggest using eval for this (it is not the solution), the problem is that eval expects complete lines of code, not just fragments.

$ma ="2+10";
$p = eval('return '.$ma.';');
print $p;

Should do what you want.

---

A better solution would be to write a tokenizer/parser for your math expression. Here's a very simple regex-based one to give you an example:

$ma = "2+10";

if(preg_match('/(\d+)(?:\s*)([\+\-\*\/])(?:\s*)(\d+)/', $ma, $matches) !== FALSE){
    $operator = $matches[2];

    switch($operator){
        case '+':
            $p = $matches[1] + $matches[3];
            break;
        case '-':
            $p = $matches[1] - $matches[3];
            break;
        case '*':
            $p = $matches[1] * $matches[3];
            break;
        case '/':
            $p = $matches[1] / $matches[3];
            break;
    }

    echo $p;
}