Calculator in PHP using switch Case

What we are building

A calculator takes two numbers and an operator (+, -, *, /) and returns the result. It is a perfect small project to practise the switch statement, which chooses one action from several based on the operator. We will build it step by step and handle divide by zero safely.

The switch logic

<?php
  $a = 10; $b = 5; $op = "+";
  switch ($op) {
      case "+": echo $a + $b; break;
      case "-": echo $a - $b; break;
      case "*": echo $a * $b; break;
      case "/": echo $a / $b; break;
      default:  echo "Invalid operator";
  }
?>

The switch matches $op to a case and runs that operation. Each case ends with break so only the matching one runs; default catches unknown operators.

The full calculator

<?php
  function calculate($a, $b, $op) {
      switch ($op) {
          case "+": return $a + $b;
          case "-": return $a - $b;
          case "*": return $a * $b;
          case "/":
              if ($b == 0) return "Cannot divide by zero";
              return $a / $b;
          default:  return "Invalid operator";
      }
  }
  echo calculate(20, 4, "/");   // 5
?>

Wrapping it in a function makes it reusable. Notice the divide case checks for zero first, returning a message instead of crashing.

Adding a form

<form method="post">
  <input type="number" name="a">
  <select name="op">
    <option>+</option><option>-</option>
    <option>*</option><option>/</option>
  </select>
  <input type="number" name="b">
  <button>Calculate</button>
</form>

<?php
  if ($_SERVER["REQUEST_METHOD"] === "POST") {
      $a = (float)$_POST["a"];
      $b = (float)$_POST["b"];
      echo calculate($a, $b, $_POST["op"]);
  }
?>

The form sends two numbers and the operator. We cast the numbers with (float) and pass everything into our calculate function.

Handling divide by zero

Dividing by zero must be handled or it errors. Our function checks if ($b == 0) first. In modern PHP you could also catch it:

<?php
  try {
      echo 10 / 0;
  } catch (DivisionByZeroError $e) {
      echo "Cannot divide by zero";
  }
?>

Either approach avoids a crash and shows a clear message to the user.

Common mistakes

• Forgetting break. Without it, later cases run too (fall-through).
• Not checking divide by zero. Always guard the division.
• Not casting form values. Cast to (float) or (int) before maths.
• No default case. Handle unknown operators with default.

Summary

• A calculator picks an operation based on the operator — perfect for switch.
• Each case does one operation and ends with break (or return).
• Guard the divide case against zero.
• Take input from a form and cast numbers before calculating.
• Use default for invalid operators.

Next, review the concept in if-else and switch, or reuse logic with PHP functions.