PHP is a terrific language for the rapid development of dynamic Websites. It also has many features that are friendly to beginning programmers, such as the fact that it doesn’t require variable declarations. However, many of these features can lead a programmer inadvertently to allow security holes to creep into a Web application. The popular security mailing lists teem with notes of flaws identified in PHP applications, but PHP can be as secure as any other language once you understand the basic types of flaws PHP applications tend to exhibit.
Top 7 PHP Security Blunders [PHP & MySQL Tutorials]