You describe one reasonable approach to building things: bad shit should blow up quickly, forcing fixes. But another reasonable way is working to make sure something reasonable happens. E.g., Postel's Robustness Principle: http://en.wikipedia.org/wiki/Robustness_principle
My understanding is that PHP started out as a noob-friendly page scripting language. For that kind of system, do-what-I-mean coding is reasonable. You're not trying to force amateurs to be pros; you're just trying to help them get something up and working. But maybe the PHP audience has shifted enough that the break-early-break-often approach is the right one these days.
I think of this as "garbage in, garbage out". The function will return a numeric value -- if you give it proper input.
On two occasions I have been asked,—"Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?" In one case a member of the Upper, and in the other a member of the Lower, House put this question. I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question.