There is no definitive naming convention in PHP, and they differ by framework:
Zend does not permit underscores
Symfony also encourages camelCase
Wordpress encourages underscores and does not like camelCase
CodeIgniter also promotes underscores
So: Use whatever your framework uses or create your own naming convention.
At least for function names and class methods, there is a one thing to consider,
but some frameworks discard it:
PHP is case insensitive in that case, so aTonalFunction() and atonalFunction()
are both calls to the same function.