Used session_start() on a site developed using PHP4.0b4-pl1, had no problems whatsoever.  Installed PHP4rc1, and page load hung completely on session_start().  PWS left this connection open, and Active Connection was never terminated.  This can be reproduced using a very simple page:

<?php
session_start();
?>

Worked on Beta 4, hung up on release candidate 1.

Platform is Win 95 OSR2 running latest version of PWS4.