Change user role of one user in functions.php

Why are you doing it in the functions, and to what purpose?

I wanted that someone would get admin rights, after filling up user details in profile (name, phone number…). Before that he wouldn’t have rights to add or edit pages and everything

That would be a big security risk, since a spammer would be able to fill in the data with junk and then have admin rights.

And you wouldn’t want to do it in the functions.php file anyway, because that’s *theme* based, when what you want is user based.

I know it is theme based.

How can I solve that diferently?

Make a plugin that, when someone logs in, checks for completion of the profile. If found, promote user levels.

And I wouldn’t do it in a million years, if you paid me.

Take the code you were going to put in functions.php and put it in a separate php file with a plugin header. then activate it as a plugin.

or put it in the mu-plugins folder.

And make sure you’re doing a check for a specific user.

The whole of wordpress is filled with functions. you are not require to stick them in functions.php.