Question on php.ini.max_execution_time

I have a PHP script that opens a socket to another (non-PHP) process. This process has been having problems of late, and can take 10 minutes or more to execute. I have max_execution_time set to 300 seconds in php.ini. However, users report to me that the PHP script does not always time-out after 300 seconds, it can be much longer than that. So, my question at last: will PHP wait to display the time-out message until after the other process completes? Or should the time-out be "hard" at 300 seconds, regardless of what the PHP script is doing (waiting for a reply from another process, doing a database query, etc.)? Anyone familiar with the low-level view of max_execution_time? Kirk