Hi @mkluczka,
To me it looks like the bitwise and operator is correct.
The error_reporting() function returns the current error reporting level which is a bit field. The $code parameter is the level of the error raised. So the if statement checks if the specific reporting level is enabled by doing the bitwise and operation.

If your warning is ignored by the if statement, you're current error reporting level probably doesn't include the E_WARNING level.
You can change it via the php.ini file by setting error_reporting = E_ALL or at runtime via by calling error_reporting(E_ALL);.
You can find the different error levels here.