Most web application the use of PHP is really something that cannot be and should not be overlooked, as it is easy to learn and most forums, CMS, blogs and other interactive platforms make use of this form.
Almost everything else featured on the internet in some way of the other uses PHP too. Although it seems to be a given fact that PHP is now actively replacing Perl for web applications it will however most definitely not replace Perl for everything.
Most web programs can be easily written using PHP but writing a web server in one is almost impossible. Therefore the web server requires the use of Perl on all counts to create an effective and usable platform.
The following are some comparative points on the merits of Perl and PHP:
Perl application using mod_perl are generally as fast is not faster that PHP application for most postings.
PHP was originally written in Perl in the form of a module before the changes were made towards improvements.
Perl has a very stable and mature database complete package which is DBI and it provides the database independent interface.
PHP is missing many features that can be considered rather important to the serious language.
PHP has a deliberate lack of advanced features and the web features that are currently included in the core language does make it easier to learn and use. Those without any programming experience can also easily use the PHP.
Superficially both languages seem rather identical and Perl is not harder to read or use than PHP.