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, JavaServer Pages (JSP), Perl, and PHP—on numerous platforms and various web servers, with varying degrees of success. PHP is the right choice for me: it's flexible, fast, and simple 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 these questions:
Can you say with absolute certainty that you will always use the same web server hardware and software? If not, look for something cross-platform that is available for all types of web servers: PHP.
Will you always have the exact same development team comprised entirely of ASP (or Java Server Pages 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 JSP 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.
Here's the bottom line: PHP is simple, so just try it! If you like it, continue using it.
It's Open Source, so help improve it. Join a mailing list and help others. If you don't like it, you're only out the money for this book, and the software can be uninstalled without rendering your machine completely inoperable.