Matthew Turland has a post to his site sharing his experience with the PHPUnit and XHProf combination...and the unfortunate result that made every test fail.

I ran into an issue recently while trying to run PHPUnit tests in an environment using XHProf. Google didn’t prove to be much help, so I thought I’d document the problem and solution here for posterity.

When I ran my tests, each failed with the same cryptic error and no backtrace: "Attempted to serialize unserializable builtin class PDO" The cause was the culmination of two rather unfortunate circumstances.

He shares the two problems that causes this issue - one being XHProf's use of globals (where its PDO connection is stored) and the other is the @backupGlobals setting in PHPUnit that's enabled by default. This makes PHPUnit to try to backup that PDO connection by serializing it but can't, hence the failure. He points out a pull request that aims to fix the issue but recommends disabling the globals backup for the time being if you don't have a need for it.