Don't forget there also is the Java world, and it's strong on Android smartphones, too. For the web that means Tomcat & Java Servlets for example.
For a judgment of the landscape, take a look at tiobe: http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html
It's kind of weird to mix full programming languages with rather limited sql or even more specific academic languages like lisp, ada, logo. But the index is made with several indicators in mind and a good indicator.
Java is in the lead, it's even more cross plattform than PHP, was there before PHP and thus is very mature, also still in development and active. You can see PHP has made a drop, mainly because C# is getting the better Java on Windows plattforms at least. and ObjectiveC is string through the Apple app market.
So PHP+Mysql is a choice nevertheless, all that I said about Java is true for PHP+MySQL, too. The learning curve is not that steep as with Java and the hosting plans include these technologies. I also do PHP rather than Java, but I'd also take a look at other databases besides mysql.
PDO is a strong class to use in PHP for access of many databases, of which I'd mention PostgreSQL. I think alone of index options and query optimisation with them.
Much more details: http://www.wikivs.com/wiki/MySQL_vs_PostgreSQL
It will be harder to find hosting for PostgreSQL, but not impossible. Depends on your location, Google is your friend of course.