The PHP servlet is not thread-safe.  This causes some versions of Tomcat to crash.  But while Tomcat 3.1 will run pages without crashing, large pages loaded very close together end up writing to the same output stream.  This is caused by the HttpServletRequest and HttpServletResponse objects being declared as instance objects within the net.php.servlet class.   Less resource-intensive pages finish processing fast enough that the output streams don't collide, but when a request is made before a large request is finished, the result of the the first response will get printed with the second.

One possible solution is that if multiple request and response objects could be stored (e.g in a hash) and matched back up in the net.php.servlet callback methods, the servlet would then be thread-safe.