We currently already set up a web-facing instance of MediaWiki (during the qunit build). Let's extend our test suite by checking the error log before and after and consider any new entries a fatal build failure.
PHP notices, warnings, errors, exceptions etc.
The primary motivation here is to catch errors triggered during load.php requests because those are almost invisible in general, and even more so when all we see is the QUnit callback through PhantomJS.
And of course it helps catching lots of other errors.
Since we already have a policy of not tolerating php errors, this should be trivial to enable. Though we do have a few known php errors, none of them trigger on regular usage (open wiki page, request load.php, run qunit tests etc.).