Bug report

I would expect phpstan to error, because strpos cannot return a negative number:
https://phpstan.org/r/7dc3f972-bcac-4aea-8bbc-ae2b063146ff

this can be a common mistake, because people might be used to javascript in which the equivalent function returns -1

real world bug caused by this typo: https://github.com/justinrainbow/json-schema/pull/641/files