When developing your own PHP scripts, be aware of security issues. Also view the OWASP Top 10 Security Vulnerabilities. Listed below are some things you can do which will make your web application more secure. For more information, visit the links above.
Always validate user input before using it to do anything in your script. Even form variables such as select boxes need to be validated to ensure that the value chosen is in the original list of values.
Always escape characters such as single or double quotes, backslashes (\), percent symbols, and other characters which could result in an unexpected use of your script.
Take extra precaution when executing commands with user input in them. It is possible for an attacker to inject a malicious command in this way.
When dealing with secure information (such as passwords), be sure to use a good password that will be hard (or better yet, impossible) to guess. Generic passwords such as 'admin', should never be used. The same goes for dictionary words. Secure passwords will be at least 8 characters long, contain at least one (1) numeric digit in them, and do not contain dictionary words. Never store passwords in plain text, always encrypt then with an irreversible encryption algorithm such as MD5.
Stay away from using default filenames, such as putting administrative functionality in the admin/ folder. Generic names such as this are easy targets for hackers to attempt to access.
When instantiating PHP on a page, always use the full <?php and ?> tags. Using shortcut tags such as <? to instantiate php could result in your script being displayed as plain text if the server configuration is changed.