I think your question would have been relevant at the PHP3 stage ,ruby & Python etc were around then as well and PERL was positively king of the web.
Since then PHP has gotten itself everywhere , it was built to solve the `web-problem` and did so , & that was before the injection of Zend etc, PHP5 OO is structured to appease java & NET heads who are beginning to realise that PHP actually does scale (albeit in a different direction) & PHP6 will close up some of the embarrasing bits of PHP.
15 million domains and counting ... one of those is the biggest on the web (Yahoo) , many of those java-heads would call 'enterprise' scale applications (which they told us PHP was incapable of since it was interpreted and not OO enough)
I think the fact that there are commonly 2 (in reality 3 ..yes I still know php3 hosts!) major verions of a language in widescale use (and supported) is itself a proof of PHP's robustness.
Until Microsoft can come up with a server to compete with $your_flavour_here) then linux based solutions will continue to rule the web (yes I know PHP is x-platform), .NET is a step forward from ASP.old but IIS remains.
I have played with and enjoyed Ruby (On Rails) but to be honest I would not want to do mass virtual hosting with it , its server bindings are (currently) too dodgy.
I don;t like to talk about CF
You can stick several hundred PHP/MySQL/PostgreSQL domains on a 1/2 decent server without issue ... only PERL can compete and if it was human readable then it would probably would appeal more to noobs.
PHP owns because you can use it from day 1 to create powerful applications with little knowledge, but unlike most similar tools it then allows you to go so much further to code far more complex applications, GUI's, shell scripting, network programming ... It will never compete with lower-level compiled anguages for device drivers etc but thats a different arena.
Anyway , if you have any scripting/programming knowledge you can pick up PHP in a matter of days , just download the manual.
<edit>I forgot Python ... I like Python , but it like perl was not originally intended for the web and it shows</edit>