Creating a simple calculator in PHP is a great way to practice your PHP programming skills. Here, I'll walk you through the steps to build a basic web-based calculator that can perform addition, subtraction, multiplication, and division. You can expand on this foundation to create a more sophisticated calculator with additional features.

Step 1: Set Up Your Development Environment:

Make sure you have PHP installed on your system. You can use XAMPP, WAMP, or MAMP for a local development environment, or use a web hosting service to deploy your calculator online.

Step 2: Create an HTML Form:

Create an HTML form to collect input from the user and display the calculator on a web page. Here's a simple example:

html
<!DOCTYPE html>
<html>
<head>
    <title>Simple Calculator</title>
</head>
<body>
    <h1>Simple Calculator</h1>
    <form method="post" action="calculator.php">
        <input type="text" name="num1" placeholder="Enter first number" required>
        <select name="operator">
            <option value="+">+</option>
            <option value="-">-</option>
            <option value="*">*</option>
            <option value="/">/</option>
        </select>
        <input type="text" name="num2" placeholder="Enter second number" required>
        <input type="submit" value="Calculate">
    </form>
</body>
</html>

This HTML form collects two numbers and the selected operator from the user.

Step 3: Create a PHP Script:

Create a PHP script (e.g., calculator.php) that will handle the form submission and perform the calculations. Here's a basic example:

php
<!DOCTYPE html>
<html>
<head>
    <title>Simple Calculator</title>
</head>
<body>
    <h1>Simple Calculator</h1>
    <form method="post" action="calculator.php">
        <input type="text" name="num1" placeholder="Enter first number" required>
        <select name="operator">
            <option value="+">+</option>
            <option value="-">-</option>
            <option value="*">*</option>
            <option value="/">/</option>
        </select>
        <input type="text" name="num2" placeholder="Enter second number" required>
        <input type="submit" value="Calculate">
    </form>
    
    <?php
    if (isset($_POST['num1']) && isset($_POST['num2']) && isset($_POST['operator'])) {
        $num1 = floatval($_POST['num1']);
        $num2 = floatval($_POST['num2']);
        $operator = $_POST['operator'];
        
        switch ($operator) {
            case '+':
                $result = $num1 + $num2;
                break;
            case '-':
                $result = $num1 - $num2;
                break;
            case '*':
                $result = $num1 * $num2;
                break;
            case '/':
                if ($num2 == 0) {
                    $result = 'Division by zero is not allowed.';
                } else {
                    $result = $num1 / $num2;
                }
                break;
            default:
                $result = 'Invalid operator';
                break;
        }
        
        echo "Result: $num1 $operator $num2 = $result";
    }
    ?>
</body>
</html>

This PHP script processes the form data, performs the selected arithmetic operation, and displays the result.

Step 4: Testing the Calculator:

Save both the HTML and PHP files in your web server's directory. Access the HTML file in a web browser, and you'll see the calculator form. Enter numbers and select an operator, then click "Calculate" to see the result.

You've now created a simple web-based calculator using PHP. You can further enhance this calculator by adding features like error handling, additional mathematical functions, and a more user-friendly interface.