This advice is incorrect I believe. PHP_OS will return the OS PHP is running on, not what the OS of the client. >From the manual: PHP_OS The name of the operating system on which the PHP parser is executing; e.g. 'Linux'. $HTTP_USER_AGENT (if you're using Apache with PHP) should do what you need. Josh Hoover KnowledgeStorm, Inc. [EMAIL PROTECTED] Searching for a new IT solution for your company? Need to improve your product marketing? Visit KnowledgeStorm at www.knowledgestorm.com to learn how we can simplify the process for you. KnowledgeStorm - Your IT Search Starts Here > > How can I detect client's OS/platform with PHP? (Win, Mac, etc.) > > with PHP_OS. $os = PHP_OS; > > See "Chapter 8 - Constants" of the manual.