It did not appear as though anything quite like it existed, so I threw together a
PHP security audit script.
Uploaded to your webserver, the script will check known PHP vulnerabilities and provide some general recommendations for securing your PHP installation.
It's still a bit rough / paranoid, though I plan to add vulnerability-specific recommendations and some filters to determine the depth of vulnerability checking in the next revision.
Informed PHP security suggestions (and general usability suggestions) welcomed.