Sorry, but wrapping a framework method just for that seems like a really bad idea and at least to me your example doesn't make any sense whatsoever. You're just unnecessarily creating a new method call that needs it's own space in memory plus all the parameters, etc. etc. The only thing you're fixing (badly, I might add) is code that was put there intentionally for illustration purposes. I could've just assigned the result of the replace method back to the withBad variable, but that would've defeated the variable name.
I mean someone will rarely ever get a hardcoded string literal where he has to replace something out of it. Why write "I'm bad men." when you could write "I'm men." to begin with? So the point is that there will always be some variable or method that gives us the original string that needs parts of it replaced, wherever the string comes from (from a file, maybe from user chat, etc. - not hardcoded). Did you consider the case that maybe original and replaced string will both be needed at some point?
Also, what you apparently didn't know, this works too:
<?php
string myString = "I'm bad men.".Replace(" bad", "");
?>
It's completely nonsensical but since you're a minimalist freak it might satisfy your desires