Hi! I've not used PHPMyAdmin for some time, but I've got a feeling that it may not allow root login from the web interface as a security feature; try creating a non-root user, giving it appropriate privileges on the database, and then see if you can log into PHPMyAdmin using your new non-root user.
Syntax would be something along the lines of:
CREATE USER 'lenny'@'localhost' IDENTIFIED BY 'password1';
GRANT ALL ON mydatabase.* TO 'lenny'@'localhost';