I upgraded Linux on this server a while back and in the course of that upgrade PHP was upgraded too, to version 5.5.9.
Since I had a modified php.ini it asked me to check over any new options. There were a few but I fixed anything that looked like it might break things. I saved the php.ini and let the upgrade go on.
Some time later I saw odd notices in the PHP error log. Usually it referred to wp-comments-post.php but files outside of WordPress raised the warning too:
PHP Notice: Undefined variable: _SERVER in ..
It turns out the fix is rather easy. Jacques Marneweck on his blog posted a solution. Simply switch off “auto_globals_jit” in your php.ini and restart Apache (if required).
From the PHP manual entry for auto_globals_jit:
When enabled, the SERVER and ENV variables are created when they’re first used (Just In Time) instead of when the script starts. If these variables are not used within a script, having this directive on will result in a performance gain.
The PHP directives register_globals, register_long_arrays, and register_argc_argv must be disabled for this directive to have any affect. Since PHP 5.1.3 it is not necessary to have register_argc_argv disabled.
The default for auto_globals_jit is “On” so next time you upgrade PHP keep an eye on the error log.