Only you can decide if PHP should be your language of choice, whether you're developing sites for personal or commercial use on a small or large scale. I can only tell you that in the commercial realm, I've worked with all the popular server-side scripting languages-Active Server Pages (ASP), ColdFusion, Java Server Pages (JSP), Perl, and PHP-on numerous platforms and various Web servers, with various degrees of success. PHP is the right choice for me: it's flexible, fast, and simplistic in its requirements yet powerful in its output.
Before deciding whether to use PHP in a large-scale or commercial environment, consider your answers to the following questions:
Will you always use the same Web server hardware and software? If not, look for something cross-platform and available for multiple Web servers: PHP.
Will you always have the exact same development team, comprised entirely of ASP (or ColdFusion) developers? Or will you use whoever is available, thus necessitating a language that is easy to learn and syntactically similar to C and Perl? If you have reason to believe that your ASP or ColdFusion developers might drop off the face of the earth, don't use those tools; use PHP.
Are memory and server load an issue? If so, don't use bloated third-party software that leaks precious memory; use PHP.
There are plenty of other questions to ask yourself when making a decision regarding a development language, and in short I can only say "Try it, you'll like it!"