It's really good to know that you are eager to learn this language at quite young age. Well, as far as knowledge is considered, age is no bar. You can learn any time any age if you have zeal of learning. I would suggest you some sites, that are quite good for a novice person to learn and know what PHP is all about, the basics, syntax's with examples and many more. PHP development is really in boom nowadays and people are hiring PHP developer
I also want to give you one personal advice, while studying this language, do not ignore your studies as they are utmost important for you presently.
Also, while learning from these sites, you can have a look at few online learning tutorials, dvd's and etc stuff.
The sites are
Lynda[.]com -> PHP tutorials
*Note - for opening these, replace the brackets, then you can easily open them.
If you still need any assistance, then feel free to repy..