PHP switch is not working -

i have made calculator in php using post , 2 params - num1 , num2. when want calculates numbers answer not showing.
should do?

here code:

if ($_post) {     $first_number = $_post['first_number'];     $second_number = $_post['second_number'];     $cal = $_post['cal'];      switch ('$cal') {         case "addition":             $result = $first_number + $second_number;             break;         case "subtraction":             $result = $first_number - $second_number;             break;         case "multiplication":             $result = $first_number * $second_number;             break;         case "division":             $result = $first_number / $second_number;             break;         case "modulation":             $result = $first_number % $second_number;             break;     }     return $result; } echo "$cal <br /> <br /> 1st number: $first_number <br /> 2nd number: $second_number <br /><br />"; echo "answer is:" . $result; 

you need remove quotes switch statement

replace

switch ('$cal') 

with

switch ($cal)