Here are some tips to create secure PHP web applications:
1. Use prepared statements and parameterized queries to prevent SQL injection. Never concatenate user input into SQL queries.
2. Validate and sanitize all user input. Use PHP's filter_var() function to validate data types and sanitize strings.
3. Use encryption and hashing functions to protect sensitive data. For example, use PHP's password_hash() and password_verify() functions for passwords.
4. Use PHP sessions to maintain state and protect from CSRF attacks. Regenerate the session ID after user login.
5. Escape all output to prevent XSS attacks. Use PHP's htmlspecialchars() or htmlentities() functions.
6. Use PHP's PDO abstraction layer for database queries instead of the outdated mysql/mysqli extensions. PDO has better security features.
7. Keep PHP updated to the latest version to patch any vulnerabilities. PHP 7+ has many security improvements over older versions.
8. Don't trust $_FILES or $_GET arrays for uploaded filenames. A malicious user could upload a .php file with a spoofed extension.
9. Use a security-focused framework like Symfony or Laravel which handle many best practices for you.
10. Follow the principle of least privilege. Don't run your PHP application as a superuser. Limit file permissions as much as possible.
11. Use password protection, SSL, and limit access to sensitive files on the server. Don't leave them publicly accessible.
12. Stay up-to-date with PHP security best practices. The PHP security documentation has more tips.
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