255 is an error, I could reproduce that same exit code by having a fatal error.
This means that somehow your error reporting is hidden, there are some possible causes for this:
- error_reporting is not defined and php reports no error at all
- An
@(error suppression operator) hides the output of the error - STDERR is redirected somewhere else (php -f somefile.php 2>/dev/null, remove the redirection)
- This could still be an internal error due to missing dependencies and that a fatal error has the same exit code as a program crash.