[eluser]CroNiX[/eluser]
Javascript and PHP are pretty much the same in that regard, as well as all other "languages". They don't like hyphens because that generally is a subtraction operation, so you can't use them in variable names, function names, etc. CSS doesn't care, as it's not a "language" and either hyphen or underscore will work fine, are are both valid/legal. Javascript and php don't care about hyphens in strings, like id's or classnames though.
