Filter_Var() is a PHP function intended to help validate and sanitize certain types of data. It can verify that an email address is in a correct format, remove harmful characters and tags, etc..
I discovered this interesting feature through Sanitize and Validate Data with PHP Filters over at NETTUTS.
The function validates and sanitizes, two things that are very much different but easily confused. Validation is the act of making sure that input is formatted correctly, while sanitization is the process of filtering out unwanted garbage that may be detrimental, such as exploitation attempts.
The following would return a string containing the validated email, or FALSE if it were malformed.