Like Perl, PHP can be used for things other than web
development. But PHP was developed specifically as a
scripting language for a WWW server, so it is has
characteristics a that make it particularly suited to
that purpose.
 
PHP is interpreted (typically through an Apache module)
by the Web server (sort of like a plug-in on the server side).
 
PHP is easy to debug, because error messages return
simply to the browser (rather than to the server's error
log). The scripting is done in the context of HTML pages,
so there's no bouncing back and forth to the cgi-bin.
When you make an error in CGI script, it makes
the server volatile, whereas PHP tends to be forgiving.
A maximum execution time of 30 seconds is the default,
so even a very badly written script will not slow down the
server for long.
 
PHP is a nicely structured language, like C or Lingo -- not as
ideosyncratic as Perl -- and therefore relatively easy to
teach.  Since it is a server-side technology, once the
script runs, it runs.  It's not like Javascript, where the
successful execution depends on the browser and version.
 
Finally, in addition to becoming a very popular  WWW
scripting language, PHP can introduce designers and prospective
webmasters to the more sophisticated techniques whereby
industrial web content is delivered.   PHP can be used,
like Cold Fusion or ASP, to develop templates that
are database driven.   Most rapidly changing content on
the web is in fact produced with some comparable scripting
technique.   PHP has the advantage of being 100% free
and popular. 

--------------------
older:

Perl can be used for things other than web development.
PHP cannot. PHP is interpreted through an Apache module
(sort of like a plug-in on the server side).

Perl can be used (through Apache's mod_perl) without
CGI -- ie. the Perl interpreter also can be imbedded in the
web server, making it about as fast as PHP.

Though this functionality is like PHP's, I find that
PHP is easier to debug, because error messages return
simply to the browser (rather than to the server's error
log). The scripting is done in the context of HTML pages, 
so there's no bouncing back and forth to the cgi-bin.  
And when you make an error in a mod_perl script, it makes 
the server volatile (this is actually something that can
be achieved with mod_perl, too, using suffix mapping instead
of a centralized CGI bin). 

Having said all that, I still like Perl.  The increasing
speeds of CPU's are offset by the slightly slower performance
of CGI.  PHP is a more C/C++/Java like language, not as
ideosyncratic as Perl, but there are positive aspects to
that ideosyncracy once it's learned.  

My gut feeling is that people would pick up the PHP faster
and it will be easier to debug.  I like it.  The question
is how much (if at all) to get into Perl if we are going
to cover PHP too.