The PHP interpreter is much like any other compiler or interpreter. It will halt on errors. It will, by default, output warnings/notices for things like accessing uninitialized variables. Like any other compiler or interpreter, you can tell it to hide warnings, or to halt on warnings, or to log them somewhere instead of outputting them. What makes this a fundamental weakness of the language, but not of, say, C, where gcc will also happily compile code no matter how many warnings it runs into?
C is intended to be low-level, like an extra fancy set of macros for writing assembly. It is intentionally not supposed to do that much for you because you need greater control to do things like writing UNIX. Nonetheless, if you use it seriously then you will find that gcc does outright refuse to compile a great many things that it recognizes as broken, rather than chugging along. There is no way to simply suppress all compiler errors because there should not be.
Nobody in this discussion has raised the possibility of writing web apps entirely in C and suppressing all compiler warnings as they did so. That is a scary idea. And it gets even scarier if we are advocating for newbies to start writing web apps that way.
