Calculator.php

<?php 
// Function to perform addition
function add($a, $b) {
    return $a + $b;
}

// Function to perform subtraction
function subtract($a, $b) {
    return $a - $b;
}

// Function to perform multiplication
function multiply($a, $b) {
    return $a * $b;
}

// Function to perform division
function divide($a, $b) {
    if ($b == 0) {
        return "Error! Division by zero.";
    }
    return $a / $b;
}

while (true) {
    // Display calculator menu
    echo "\n--- Basic Calculator ---\n";
    echo "1. Add\n";
    echo "2. Subtract\n";
    echo "3. Multiply\n";
    echo "4. Divide\n";
    echo "5. Exit\n";
    
    echo "Choose an option (1-5): ";
    $choice = trim(fgets(STDIN));
    
    if ($choice == 5) {
        echo "Exiting the calculator. Goodbye!\n";
        break;
    }
    
    // Get the first number
    echo "Enter the first number: ";
    $num1 = trim(fgets(STDIN));

    // Validate first number
    if (!is_numeric($num1)) {
        echo "Invalid input. Please enter a valid number.\n";
        continue;
    }
    
    // Get the second number
    echo "Enter the second number: ";
    $num2 = trim(fgets(STDIN));

    // Validate second number
    if (!is_numeric($num2)) {
        echo "Invalid input. Please enter a valid number.\n";
        continue;
    }
        
    // Convert input to numbers
    $num1 = (float)$num1;
    $num2 = (float)$num2;

    // Perform the chosen operation
    switch ($choice) {
        case 1:
            echo "Result: " . add($num1, $num2) . "\n";
            break;
        case 2:
            echo "Result: " . subtract($num1, $num2) . "\n";
            break;
        case 3:
            echo "Result: " . multiply($num1, $num2) . "\n";
            break;
        case 4:
            echo "Result: " . divide($num1, $num2) . "\n";
            break;
        default:
            echo "Invalid option. Please choose between 1-5.\n";
    }
}
?>