Preforming math stored in variables

Question

I have 3 variables like this: $first = 2; $second = 5; $operation = '*';

how can I programaticly assign the solution to this math problem to the $answer variable? I have tried eval(), but that does not work.

Answer 1

function add($a, $b) {
    return $a + $b;
}

function multiply($a, $b) {
    return $a * $b;
}

function divide($a, $b) {
    return $a / $b;
}

$operations = array(
    '+' => 'add',
    '*' => 'multiply',
    '/' => 'divide',
);

$a = 2;
$b = 5;
$operation = '*';

echo $operations[$operation]($a, $b);

Answer 2

eval() should work perfectly fine for something like this. Remember that eval() returns NULL though, unless you tell it to return something.

<?php
$first = 3;
$second = 4;
$operation = "*";

$answer = eval('return '.$first.$operation.$second.';');

echo $answer;
?>

Answer 3

It works fine for me:

var_dump(eval("return $first $operator $second;"));

But eval is evil. You should better use a function like this:

function foobar($operator)
{
    $args = func_get_args();
    array_shift($args);
    switch ($operator) {
    case "*":
        if (count($args) < 1) {
            return false;
        }
        $result = array_shift($args) * array_shift($args);
        break;
    /*
        …
    */
    }
    if (count($args)) {
        return call_user_func_array(__FUNCTION__, array_merge(array($operator, $result), $args));
    }
    return $result;
}
var_dump(foobar("*", 3, 5, 7));

Answer 4

If you are planning on doing this calculation often, I would strongly recommend against using the eval method and instead use the method Ionut G. Stan posted above. I know it's more complicated, but each time you run eval, PHP loads the compiler engine all over again, and so it has a high overhead cost. You'll get a major performance increase (around 10x) if you use the function dispatch approach Ionut showed. Take a look at the comments on the eval function in the PHP manual and you'll see other people who have also been saying this: http://www.php.net/eval

I was once using eval hoping it would enable me to construct a fast templating mechanism for a web application, but calls to eval were very slow, and when I realised that eval was causing it I switched to a different approach. The speed gains were phenomenal.