Single Quotes vs Double Quotes

Hi everyone, Could somebody please explain to me the difference between single quotes and double quotes in PHP. It seems like they can be used interchangeably a lot in PHP but not always. For example: This works: header('Location: http://somedomain.com') This does not work: header('Location: $url') This works: header("Location: $url") Why??? Sorry if this has been asked before, but I didn't find a suitable answer in the archives. TIA. Mike __________________________________ Do you Yahoo!? Yahoo! SiteBuilder - Free, easy-to-use web site design software http://sitebuilder.yahoo.com