C++ is the farthest thing from being "Windows Only"...

As for Javascript, it's a web only technology, powerful when you get into the whole AJAX thing.

PHP and MySQL are again web only, this buletin board's coding language is PHP and MySQL is the database that holds all forum content (posts).

If you're one of these "oh i want to hack every website I see" then I'd highly recommend you become very well-versed in web tech, such as PHP/MySQL/Ajax items as you'll know the in's and out's of what's there and how things work to be able to check people who don't secure their coding.
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