PHP is a flexible language with hooks into just about every API offered on the machines on which it runs. Because it was designed to be a forms-processing language for HTML pages, PHP makes it easy to use form data sent to a script. Convenience is a double-edged sword, however. The very features that allow you to quickly write programs in PHP can open doors for those who would break into your systems.
PHP itself is neither secure nor insecure. The security of your web applications is entirely determined by the code you write. For example, if a script opens a file whose name is passed to the script as a form parameter, that script could be given a remote URL, an absolute pathname, or even a relative path, allowing it to open a file outside the site’s document root. This could expose your password file or other sensitive information.
Web application security is a young and evolving discipline. A single chapter on security cannot sufficiently prepare you for the onslaught of attacks your applications are sure to receive. This chapter takes a pragmatic approach and covers a distilled selection of topics related to security, including how to protect your applications from the most common and dangerous attacks. The chapter concludes with a list of additional resources as well as a brief recap with a few additional tips.