The consensus seems to be that one should follow the convention of the platform they're developing for. See:

Underscore or camelcase?

Naming conventions: camelCase versus underscore_case?

However, PHP doesn't seem to strictly follow any convention internally (no surprises there), even for methods and functions (e.g. mysqli::set_local_infile_default,
PDOStatement::debugDumpParams); however, underscores seem to be dominant in function names. 

However, what I couldn't find was this: what's the dominant naming convention for variables in PHP?

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.
