I understand that many php scripts can run concurrently and that the actual limit is determined by hardware. However, I have written a script that benchmarks a mysql database application and am finding that only two instances of the script will run at a time when executed via an http request.

When the script is executed four times simultaneously from four different shell sessions, all four instances execute at the same time and finish at approximately the same time.

When the script is executed simultaneously from four different browser sessions on the same client machine, only two instances execute at a time. The third and fourth instances wait for the first and second instances to complete before executing. Why is this? Is this a hard limit or a configurable limit? Is the limit related to php an/dor Apache?