Single Underscore (Drupal)
“If you need to define global variables, their name should start with a single underscore followed by the module/theme name and another underscore.”
“For methods on objects that are declared with the “private” or “protected” modifier, the first character of the method name must be an underscore. This is the only acceptable application of an underscore in a method name. Methods declared “public” should never contain an underscore. ”
“For instance variables that are declared with the “private” or “protected” modifier, the first character of the variable name must be a single underscore. This is the only acceptable application of an underscore in a variable name.”
Double underscores (__) means nothing in PHP since it is not one of the reserved PHP keywords.
It’s meaning really depends on the project. Do not be surprise when it means something in wordpress and something else in zend framework or cakephp.