Ajax-php question about global variable modification (for beginners)

After a user logs on, the user information is stored in the global variable global $ user of php. edit the user, modify the user name, and click Save. the user name is saved to the database, however, the user name displayed on the page has not changed. I would like to ask how the ajax callback function can modify the value of php's Super global variable? Ajaxphp

After a user logs on, the user information is stored in the global $ user variable of php,

Edit the user, modify the user name, and click Save. the user name is successfully saved to the database, but the user name displayed on the page is not changed,

I would like to ask how the ajax callback function can modify the value of php's Super global variable?