Auto-detect pc/mobile browser then redirect

We have created a website for a client. The client now wishes to have a? iPhone (and most other leading mobile browsers) optimised version of the same site. We have created? a basic HTML? version? (following specific rules about sizes etc) of his existing website but need a script to direct users who are using a mobile phone to this version. The client has specified that the redirection must take place automatically when a user lands on the homepage of his existing website and does not want a function such as - "if you are viewing this website on a mobile phone, click here". So we are looking for a Script that activates when a user visits the homepage of the existing website, checks what kind of browser is being used then if its a standard pc browser takes no action and if its a mobile phone browser re-directs the user to a seperate area of the server where the iPhone optimised website files are found.