PHP is a tool for a very specific job, namely database connectivity with websites. This is the tool you can expect to use in the real world. Your other options are ASP or Perl. ASP is a MS solution and that's good enough reason for a lot of people to avoid it (it's also a good enough reason for a lot of people to use it). I find ASP to give me a headache, so I try to avoid it. Perl requires mod_perl and ePerl to get it to function similar to PHP. These are extra things that have to be installed and more places for something to go wrong and makes Perl not as well suited as PHP for this task.

For a "web design" or "interactive websites" class, PHP is the correct language to focus on. For another class, say "programing for system administration" or anything without the word "web" in it, the Perl would be a much better choice.

Perl is a jack-of-all-trades language, meaning it can do a lot of things at least mediocre or better. PHP is a specalized language, meaning it can do one thing really well, but not much else.

Think of it this way...
I'm a jack-of-all-trades. If given enough time, I can build a house from the foundation up to the roof and not have it fall down. However it will probably just barely pass the building codes. If I want the job right, I'll hire someone to lay the foundation, someone else to build the frame of the house, someone else to put the plumbing in, someone else to do the walls, someone else to put the shingles on, and someone to do the roof. All these people are specalists who do one job and one job extremely well. They however would be lost if they had to do another aspect of building the house.

That's what PHP is. It does one job and does it extremely well, but it can't do anything else. Perl on the other hand can do a lot at least well enough to get it pass the inspectors.

If you're going to be building websites, PHP is the tool you should use. If you're going to be doing something complex in the background, that's where Perl comes it.