Today I had a weirdly failing unit test. PHPUnit complained about the data provider being invalid, with a very vague error:

Unsupported operand types: string * int

This was weird, because I wasn’t doing any calculations in the dataprovider.

That made debugging annoying as well. If I’m not doing any calculations, why this error?

It turned out a typo was the cause. The weird thing though: There was no * in there. The typo was:

$errorString -= 'Missing required data key: "%s".';

See the -= there? That was the issue. It’s still unclear to me why I got the error I got, but if anyone runs into this error, be aware that the error may imply more than just *.