It's one of the more common languages because of it's power -- that is not to be argued. Though I do believe that developers interested in the long haul of the industry must familiarize themselves with other languages and frameworks as well (such as .NET).
At some point in your career as a web developer, you will be asked to program in something other than PHP. Being an asset includes the ability to adapt to different situations well. As a result, it is strongly recommended that web developers not limit themselves to a single language but rather open themselves to constantly learning new things and discovering the power in knowing all of the options. :-)
We moved away from PHP some time ago and only use a couple things in PHP. These days it is all about ColdFusion (CFML) - If you like PHP and want to learn a second language would highly recommend ColdFusion!