PHP Manual: Sets the default domain

string textdomain(stringnull $domain)

This function sets the domain to search within when calls are made to gettext, usually the named after an application.

domain: The new message domain, or null to get the current setting without changing it

If successful, this function returns the current message domain, after possibly changing it.

Throws a ValueError if domain is the empty string.

Version	Description
8.4.0	Now throws a ValueError if `domain` is the empty string.

Note:
The textdomain information is maintained per process, not per thread.