In his latest post on the ThinkPHP blog, Andreas Uhsemann steps through a simple conversion he made from using a MySQL database with his application to using PDO on an Oracle system.
At first, one would notice that nothing was happening at all. Just a white screen, and no entries in the error log. Even if one turns on error messages explicitly (you never know the current php.ini settings) with error_reporting(E_ALL), nothing is changing.
He had the connection all set up (manual tnsnames entry and all), but there was something wrong and he couldn't see it. After changing the error reporting level (to no avail), he went for PDO's error handling. The result was an "invalid character" error from the extra semi-colon on the end of the statement.