Ensuring Security in PHP Applications: Common Pitfalls and Solutions

How can developers secure their PHP applications against common vulnerabilities such as SQL injection, cross-site scripting (XSS), and cross-site request forgery (CSRF)? Discuss strategies like using prepared statements, input validation, and output sanitization. Additionally, what are some best practices for managing user authentication, password hashing, and session handling? How can developers ensure that their applications remain secure over time with regular updates and by monitoring potential security flaws? What tools or libraries can assist in maintaining the security of a PHP-based application?