Setting process priority

Hi there, here is my problem I'm using php to generate a dynamic pdf document and when ready send it with email. Since it takes few minutes to generate I start a new process with popen that makes the pdf like this popen("./pdf.php \"".$outputarray."\" > /dev/null &", "r"); pdf.php then generates it and everything is ok but this new process takes so much cpu that the site is halted for these few minutes. My question... is there a way to set pdf.php-process to a lower priority? I've searched everywere and couldn't find it. Please reply in email since I will miss it otherwise Gunnar Steinn gsm@hi.is