I recently performed a MySQL upgrade on Windows Server 2008R2.

I moved from MySQL 5.1 to MySQL 5.6.22 (Community Edition).

(NOTE: PHP is currently 5.3.0)

Things went just fine for all my WordPress installs (no problems on cutover); however, all my Drupal 7 sites are now generating 500 errors (coming from php-cgi.exe via php5ts.dll from what I can tell).

I've checked database credentials, time zone defaults, and haven't changed IIS settings, folder permissions, or anything of that nature.

The one difference that stands out, is that Drupal uses PDO for access, and I'm wondering if there is a potential issue there (re: versions ... since all these sites worked fine under MySQL 5.1).

PDO seems to be showing version 5.0.5-dev (rev 1.3.2.27).

I can't locate anything that leads to identification, troubleshooting or resolution.

(I'm off to try to replicate with just a simple PDO test.)

Has anybody encountered this and/or have documentation that might help isolate/resolve things?

NOTE: The closest I've come is to somebody using PDO drivers for MSSQL (not MySQL) where there was a faulty Drupal layer on top of PDO. Details here - http://forums.iis.net/t/1181446.aspx?PHP+5+3+8+Problems