What does the double underscores in these lines of PHP code mean?
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.
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:
Note: You may use the underscore character ‘_’ as an alias to this function.