What best practices should be followed when defining and calling functions for input validation in PHP scripts?

When defining and calling functions for input validation in PHP scripts, it is important to follow best practices to ensure the security and integrity of your application. This includes validating all user input to prevent SQL injection, cross-site scripting, and other security vulnerabilities. It is recommended to create separate functions for different types of input validation, such as validating email addresses, passwords, and numeric values. Additionally, always sanitize and escape user input before using it in database queries or outputting it to the browser.

// Function for validating email addresses
function validateEmail($email) {
    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        return false;
    }
    return true;
}

// Function for validating passwords
function validatePassword($password) {
    if (strlen($password) < 8) {
        return false;
    }
    return true;
}

// Example of calling the validateEmail function
$email = $_POST['email'];
if (!validateEmail($email)) {
    echo "Invalid email address";
}

// Example of calling the validatePassword function
$password = $_POST['password'];
if (!validatePassword($password)) {
    echo "Password must be at least 8 characters long";
}