User A was getting 503 errors. When we looked at his error log, we found many entries that looked like this:

2014-01-28 10:05:51.751 [INFO] [lsphp5] PID: 45143, add child process pid: 45175, procinfo: 0x15656f0
2014-01-28 10:05:51.790 [INFO] Remove pid: 45175, exitcode: 255

For some reason PHP processes were quitting right after they started.

We then looked through his stderr.log and found the following errors corresponding to the time stamp 10:05:51:

2014-01-28 10:05:51.789 [STDERR] PHP Fatal error: Directive 'register_globals' is no longer available in PHP in Unknown on line 0
2014-01-28 10:05:51.828 [STDERR] PHP Fatal error: Directive 'register_globals' is no longer available in PHP in Unknown on line 0
2014-01-28 10:05:51.867 [STDERR] PHP Fatal error: Directive 'register_globals' is no longer available in PHP in Unknown on line 0

PHP is having trouble with the directive register_globals. When the user removed the directory from his php.ini, the 503 error was solved.