Finding all the security flaws and risks associated with in a piece of code could be difficult as it needs in-depth analysis of what actually the code does. Though simple security bugs in general follow certain style of coding that could be easy to find with search tools.
Feel free to grab more information on the types of security vulnerabilities and how you can find it quickly by using grep program in PHP coding. Visit http://www.phpclasses.org/blog/post/206-Using-Grep-to-Find-Security-Vulnerabilities-in-PHP-code.html.