Application servers / job priorities ?

Hi, I am developing a portal that will allow users to perform various calculations on their data sets. These calculations (handled by C / Fortran programs) will be run as external processes (and may take up to a few hours to complete). I am wondering if there is something that I can use (either in PHP or as another external process) to act as an application server i.e. allow multiple users to submit requests for calculations, but place these requests in a queue - perhaps associating each calculation with a priority, and handling each request as appropriate. Can anyone suggest anything that may faciliate this? Any links/hints would be greatly appreciated. Thanks very much in advance. - Best regards, Lee