Files that Starts with Underscores in PHP

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.”
http://drupal.org/coding-standards


Zend Framework

“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.”
http://framework.zend.com/manual/en/coding-standard.naming-conventions.html

 

Double Underscore
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.
(WordPress)
http://codex.wordpress.org/Translating_WordPress#Localization_Technology

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