how to perform operations with operator symbol saved in a variable

Question

guys this is a simple question for most of you probably. but im confused on how can i perform operations with operator symbol saved in a variable. Example.

$first=5;
$second=5;
$operator="+";

$result=$first.$operator.$second;

echo $result;

but $result will just print 5+5. i want it to perform the operation.

my idea is to put it all operations in an if condition -> if($operator == '+'){add the first and second operand}. any other ideas guys?

You have to use `eval()`. That's usually an indicator that you should be doing something different. I suggest reviewing your strategy. — John Conde, Mar 15 '13 at 13:33

score 8 · Answer 1 · answered Mar 15 '13 at 13:53

Instead of using eval(), you may try a custom function with a switch() inside:

$first = 5;
$second = 3;
$operator = '+';

$result = mathOp($operator, $first, $second);
echo $result;

function mathOp($operator, $n1, $n2){
    if(!is_numeric($n1) || !is_numeric($n2)){
        return 'Error: You must use numbers';
    }
    switch($operator){
        case '+':
            return($n1 + $n2);
        case '-':
            return($n1 - $n2);
        case '*':
            return($n1 * $n2);
        case '/':
            if($n2 == 0){
                return 'Error: Division by zero';
            }else{
                return($n1 / $n2);
            }
        default:
            return 'Unknown Operator detected';
    }
}

+1 check numeric value. Nice & simple answer with more security. — Tony Stark, Mar 28 '13 at 09:29

score 1 · Answer 2 · edited Jan 05 '14 at 16:48

1

You have to use eval() which executes a PHP operation.

$first=5;
$second=5;
$operator="+";

$term = $first.$operator.$second;
eval("$result = " . $term);

echo $result;

But be careful with eval it executes every PHP function. (Even exec..)

edited Jan 05 '14 at 16:48

BenMorel

34,448
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answered Mar 15 '13 at 13:53

idmean

14,540
9
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For extra security you may check if $first and $second [are integer](http://www.php.net/manual/en/function.is-int.php) and check if $operator is a valid operator `if(in_array($operator, array('+', '-', '*', '/', '%')))` then execute the eval function – HamZa Mar 18 '13 at 10:15

score 0 · Answer 3 · edited May 23 '17 at 10:27

Eval is one solution, but there are others. If your search SO for math parser, you will find many answers. This one is a good example.

So basically, your choices are: eval() with some validation first, or a custom math parser that does it for you, such as evalMath.

If your are looking only for simple operations (+, -, *, /) and two operators, use a validation function for your parameters (numbers only, known operators only) and use eval. If your want to be able to parse complex operations (parenthesis, variables...) then go with a parser.

score -8 · Accepted Answer · answered Mar 15 '13 at 14:33

-8

You are not far off with your own syntax, when you use

$first.$operator.$second;

You are literally adding the strings together, you need to use the '+' operator to successfully use your method. See below code, tried and tested

    $first = 5;
    $second = 5;
    $operator = '+';

    $result = $first+$operator+$second;

    echo $result;

?>

answered Mar 15 '13 at 14:33

Martin

92
8

6

What you're doing here is just adding two variables like usual. Let me explain: `$result = $first+$operator+$second;` PHP checks the value of $first which is 5, then PHP checks the value of $operator, but oops it's a string, so PHP converts it to a number which is 0 `echo intval('+');`, and then PHP checks $second which is 5 so the equation becomes 5+0+5 = 10. So for example if you do `$operator = '*';` It will always return 10. – HamZa Mar 16 '13 at 22:22

how to perform operations with operator symbol saved in a variable

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