What are some key principles of professional programming that PHP developers should follow to enhance the quality of their code?

One key principle of professional programming that PHP developers should follow is to adhere to coding standards and best practices. This includes using meaningful variable names, organizing code into functions and classes, and properly documenting the code for future reference. Another important principle is to write modular and reusable code. This involves breaking down complex tasks into smaller, more manageable functions or classes that can be easily reused in different parts of the application. Additionally, PHP developers should prioritize security in their code by implementing proper input validation, sanitization, and escaping to prevent common vulnerabilities such as SQL injection and cross-site scripting attacks.

// Example of adhering to coding standards and best practices
function calculateTotal($price, $quantity) {
    return $price * $quantity;
}

// Example of writing modular and reusable code
function getUserFullName($user) {
    return $user['first_name'] . ' ' . $user['last_name'];
}

// Example of prioritizing security in code
$username = $_POST['username'];
$username = filter_var($username, FILTER_SANITIZE_STRING);