How can you display the current user's name in the header region?
Using php filters is not recommended.

Instead create a User view for it.

Steps to creating a user view:


Go to Admin -> Structure -> Views
Click on "Add new View"
Now simply create a User view.





Click on Continue and edit button.
Add a contextual filter for uid.





Click on apply button. And the filter value as "User Id from logged in user".





Click on Apply button.
In the field, add the name of the user as the field to display. If you want to show more things just add more fields to it.





Save the view and place the block in the header region.