Perhaps because it isn't really, or only in a limited fashion (with lots of "extensions" disabled, giving only a very lite version of php)?
I doubt they're being more clever, it's just that their product/language is lots simpler.
I will now quote http://email@example.com/msg01553.html
Description Perl PHP
Namespace management Y N
File-scope globals Y N
Regular expressions Y Y
OOP support Y N(2)
Run time code mutability Y Y
Extensive lib collection Y Y(4)
Closures Y N
anonymous subroutines Y N
automatic code creation Y N
(2) PHP support for objects is not much different than C++ support for
(4) Not quite extensive, but it's getting there - PEAR is only a year or
Since there is no way in perl to prevent a program from modifying `import' among other things, you can't share a perl process among different users.
Also, perl's regex engine is a "beast", and there exist some race conditions (sorry, I don't got a link handy to a discussion), and PHP just uses PCRE, which is Perl Compatible RE, but not quite Perl re ( and lots of features in the current perl re engine are **experimental**)