7- Disable a block or edit it without access to the admin interface I activated php code in a block in the header, and I put "echo $user;" instead of "echo $user->name;".

Now I get an error on page load and can't edit the block to correct the mistake.

If I could change the active admin theme back to bartik, that would also solve the problem.

Any ideas welcome. Thanks

Edit: ok I think I solved by deleting my custom theme folder. Good that the block was only enabled for the custom theme.
If you have devel installed, you can edit the default theme by editing theme_default variable.
http://yoursite.com/devel/variable/edit/theme_default

Or deactivate the block in your database

update blocks set status = 0 where delta = 'block';