Restart Process Or Set PID

[Anon]

Hi,

I use a form to sent some settings to a php script which runs ImageMagick's convert with these options. If I submit the script more than one time it runs a new process everytime the for was sent. But I want just 1 process for every SessionID, which is also submitted.
How do I get the PID for 'convert'-process or how do I set it, and how do I restart this process if the script is submitted again.

Thanks.

Noizee