My first idea was to learn Java. Java, which is cross platform like PHP, is a compiled language, which means it naturally runs faster due to the fact it doesn't read through the raw code each time a program is run. To my disappointment though, Java isn't as straightforward to develop for as PHP. This is mainly due to my habits, as I use raw text editing for my projects, rather than an IDE. I am sure if I spent the time to learn Java as well as I know PHP, then this would be a different story, but unfortunately I don't have the time. So ultimately, Java is not the option I chose for building the back end to my web apps. On a side note, I also tried installing Apache tomcat to run .jsp files from my server. Since this failed, this was the main reason I decided not to put more time into learning Java, maybe in the future.
Secondly, I decided to go for Python. Python isn't compiled language, but due to it's simplicity, is used for many real-time applications and web apps. Personally, I don't like the syntax and find it a bit dry to work with. Also I couldn't get mod_python working on my server, which meant I couldn't run web apps via Apache. This is another project for a later date.
I then came to realise, I know PHP extremely well. I could write any application straight off without any guidance, documentation or code suggestion. Why should I learn a new language? My answer is simple. I won't! I recently came across a project called HotSpot which is used by Facebook internally to make PHP work for their billions of monthly page views. Their initial answer was to code PHP extensions using C++, which runs as native code, speeding up many aspects of their applications. They then went on to build a project to convert PHP to C++, so that PHP can essentially become a compiled language. People had done this before on a small scale, but never converted a site the size of Facebook.