Hello ! How to write a programme in PHP which executes a routine at a given time ?

Dear all , Hello ! I am a student and going to write a suit of programme . The producers as follow producer 1. User A add an new appointment A ( "open Winamp at 2:30 on computer B") through IE in his local computer A . producer 2. Computer A send the message to server A through http . The Servlet / ASP.net which locates on server A receives the message and send message to computer B (IP or domain name of computer B is given , named "www.computerB.com") . producer 3. Computer B receives the message from server A through http at 1:01 . It is expected to implement the new added appointment A ( "open Winamp at 2:30 on computer B" ) at given time . I know what to implement the producer 1 and 2. However , I don't know how to implement producer 3. Do you mind to tell me how to make a PHP programme to execute a centian routine at a given time ? Thank you !