Is there some PHP line to detect mobile phone?

Mobile
1

Is there some good PHP line to detect mobile phone as used by Amazon?
They recognise mobile phone and show mobile content.

2

$_SERVER[‘HTTP_USER_AGENT’] will return user agent of the browser and OS name/version. You can check it so see if it is a mobile platform and send different content to them.

3

or $_REQUEST[‘HTTP_USER_AGENT’] but not on all servers. the $_SERVER[‘HTTP_USER_AGENT’] will work on all servers