My Ubuntu server is configured with Apache 2.2.8 and PHP 5.2.4-2ubuntu5.18 in FastCGI mode. Everything works well, except I am seeing 500 errors that only seem to come from bots accessing the server.. for example (access.log):

x.125.71.104 - - [16/Nov/2011:10:27:39 +1100] "GET / HTTP/1.1" 500 41377 "-" "Mozilla/5.0 (compatible; Baiduspider/2.0; +http://www.baidu.com/search/spider.html)"

x.40.103.239 - - [16/Nov/2011:11:05:56 +1100] "GET / HTTP/1.0" 500 14717 "-" "Mozilla/5.0 (compatible; mon.itor.us - free monitoring service; http://mon.itor.us)"

x.249.67.114 - - [14/Nov/2011:20:57:17 +1100] "GET / HTTP/1.1" 500 101 "-" "Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)"

x.55.39.85 - - [14/Nov/2011:19:31:06 +1100] "GET / HTTP/1.1" 500 7032 "-" "msnbot/2.0b (+http://search.msn.com/msnbot.htm)._"

It is my understanding that a 500 error will be thrown when the PHP process fails to respond to Apache, which could be caused by a fatal PHP error or if PHP runs out of processes.. so my assumption is that either the bots are hitting the server too hard, killing the PHP processes, or something in the request header from bots is causing a fatal error in my PHP script? If anyone can offer advice on this it would be greatly appreciated!

Ryan