No one here will too-seriously argue with you, to be sure... yes, PHP is an appropriate solution for “a helluva lot of projects, big and small, whose success speaks for themselves.”

It's just that ... well ... ;-) ... none of us would ever want to (again... yuck!) work on such a project!   ;-) ;-)
... (unless someone had green money in their hands... then, we'll talk)

“The elegant beauty of Perl,” in such situations, is:   TMTOWTDI. You've now stumbled-upon at least three of the many contenders that are all “bringin’ home the bacon” every nanosecond in the Internet world. What we hope that you have noticed by now is:   they're all different, yet they're all Perl.

• Wanna do it “the PHP way?” Cool! Still Perl!
• Wanna do it “another way?” Cool! Still Perl!

You are absolutely correct that PHP is “an engineering-wise valid choice” for thousands of web-deployments. The software engineers who designed it and built it are top-notch, and their choices were unquestionably valid for what they intended to do. There is a demand for what they set out to do, and “they did a helluva job.” Every engineering project is a product of its appointed design parameters, at the instant in internet-time when those choices were made, and thereafter it should be judged by nothing else.

“Know PHP?” You'd be a fool not to.

“Know Perl?” You'd be unworthy to set foot upon this planet   ;-)   not to.