"Waste time on PHP" because it's not wasted time.

PHP is a language that has its uses, and its design is such that it can accomplish some (yes, narrow) tasks much more easily than Perl can. I increasingly find, in my own web development work, that a given project is best served by a combination of PHP and Perl. Perl gets used to manage the files for a dynamic website, and PHP gets used to render output to the browser, for the most part. There is some crossover, and even some interaction between the two.

Neither language is as absurdly useless as something like Whitespace. Each has its uses, and each is better at some things than the other.

I would recommend learning both quite thoroughly for any web programmer. If, however, you are only going to learn one (or if you don't really do much/any web programming), I definitely recommend Perl over PHP as the more versatile, and ultimately more powerful, tool.