Stefan Esser of the PHP 4.3 Quality Assurance Team released the very first version of his patch set aiming at hardening PHP in various ways:

Hardened-PHP adds security hardening features to PHP to protect your
servers on the one hand against a number of well known problems in
hastily written PHP scripts and on the other hand against potential
unknown vulnerabilities within the engine itself.

Sounds interesting, and thinking about security a bit more obviously has to be a part in helping PHP coming of age.
The current patches are against PHP 4.3.6, but, according to their author, will have been ported to PHP5 until the release of 5.0
I don't have PHP4 running any more, so I won't test this yet, but perhaps you have the time… test reports are highly welcomed.

Edit: Looks like it's under heavy use already.