PHP is a free programming language used primarily in web applications (it is used in the ever-popular PHP-Nuke web portal software). It boasts compatibility with countless other protocols, such as CGI, and other programming languages, such as C++ or Python. Over decades of development, it has grown into an imperative, object-oriented programming language that can be used for applications ranging from general to specific.
Make Your Mark, However You Choose
One of PHP's greatest virtues is its compatibility with other programming languages, and indeed, even with concepts. If you want to do something with the language, you can likely do it. Any website idea, or making your website work with any other piece of software in any other programming language: PHP likely can do it. If it can't by default, someone out there probably has tips on how to make it the thing you want. But this compatibility means the language is large and sometimes slow.
Learn What You Know
In summary, PHP is a good choice for anyone with a little bit of existing programming experience looking to become a web developer. It is not necessarily a good first programming language, as web development has intricacies that PHP does little to salve. But if you are willing and able to learn the language, and accept that it is writ large, both in terms of its scale and its footprint, you will probably not be disappointed.