Why set a second parameter in the translate function?

Question

The second parameter in the translate function is optional, but I always see people use it and some don't.

So I want to know if I should use it or not and if yes, how I will use it later.

I asking this because I'm new to this, and I'm using e() and _() right now in my theme, and I'll learn how to translate it when I finish.

Thanks in advance

Answer 1

Welcome to WPSE Peter,

The second argument is a translation domain. This is essentially where the translation for the first argument can be found. By default it checks the WordPress language folder - but if you are creating a custom theme or plug-in, you cannot rely on the translation for that string to be there (after all the WordPress language folders only contain translations for WordPress, not your plug-in/theme).

The setup is different for plug-ins, but to register a text domain for themes you use load_theme_textdomain:

add_action('after_setup_theme', 'my_theme_setup');
function my_theme_setup(){
    load_theme_textdomain('my-unique-theme-domain', get_template_directory() . '/languages');
}

The first argument is the domain name, which is passed as the second argument in _e() and __(). The second is where the translation files can be found.

See Codex for _e() and __().