What does the double underscores in these lines of PHP code mean?
$WPLD_Trans['Yes']=__('Yes',$WPLD_Domain);
$WPLD_Trans['No']=__('No',$WPLD_Domain);
Looks like you're using Wordpress - wp-includes/l10n.php defines __ as a function that translates a string (similar to gettext and its alias, _ but with an optional parameter for explicitly specifying a domain).
Strictly speaking, it means nothing in PHP as it is not a pre-defined function. However, in many frameworks, like CakePHP, and other libraries the double underscore is a function used for translating strings based on the user's language/locale preference.
As mentioned it is generally used for translating text between languages but really it is used in the same context as any function call.
testfunction();
is no different then
__();
WordPress documents it's __() function, part of the localisation technology here: https://make.wordpress.org/polyglots/handbook/translating/working-with-core/#localization-technology
It is difficult to find documentation because __(), __('') or __("") is not very searchable, double underscore and parentheses (round brackets) are keywords to use.
A similar or third-party GNU gettext based implementation:
http://www.php.net/manual/en/function.gettext.php
Note: You may use the underscore character '_' as an alias to this function.
来源：https://stackoverflow.com/questions/1777131/double-underscore-in-php