Note: I’m ultra busy and so I haven’t read all the other responses, so I hope I am not repeating ideas.
I am almost tempted to say that I agree with the spirit of what was said, but I would not put my name to the characterization that your professor/teacher gave.
It’s incontestable that PHP is an extremely popular programming language. That said, I would still count it among “domain-specific” programming languages, and if I were trying to convey more transcendental (sorry!—background in philosophy here) concepts, I wouldn’t use PHP. PHP is among the so-called ”Turing-complete” programming languages (i.e., it’s not AppleScript—it’s suitable for any programming problem), but I think it has so much Web-specific design (just think of the PHP prolog and how XML-influenced it is) that it could distract from the core programming discipline.
PHP isn’t a hobbyist programming language: many professionals use it. In fact, I’d say Python 3 is more of a “hobbyist” language than PHP. That just sounds like a rather humbug cackle from the ivory tower rather than a statement about reality.
And as for the “easy” part. Boy, I am really beginning to dislike your teacher. Programming shouldn’t be needlessly byzantine or difficult. A good language is one that lets you present your ideas clearly, not cryptically. I don’t find PHP easy; in the sense that I do not find it easy to formulate even semi-difficult ideas with anything approximating clarity. I find it much easier to do this in, let’s say Python, or C with Literate Programming.
Summary: PHP is popular, but not exactly elegant for treating the core of computer science. Your teacher sounds like a dbag.