Throwing 500 error, but nothing logged in error logs or apache logs?
It's likely environmental/configuration related and unfortunately some hosts do some things to make it really hard to debug stuff like this.

Off the top of my head, some common ones are if you've got something like Suhosin or mod_security installed, then they have their own configuration limits on top of the usual PHP/Apache limits that might be coming into play.

Or if you're running nginx/php-fpm as a proxy in front of Apache, you might be bumping into its configuration limits.

Or we've seen some hosts &lt;cough&gt;RACKSPACE&lt;/cough&gt; that have hard-coded execution times of 30 seconds at the router/firewall level and there is absolutely nothing you can do to get around it, other than the obvious thing, which is to never use Rackspace.