How can I modify the values in variable table using phpmyadmin?
As stated in comments serialized string variable lenght must match the integer prefixed, eg

s:20:"example@example.info";


drush vset is alot better for this kind of adjustements.

That beeing said, for users with drush 4.5 where modifying more complex variables is not available (use variable-set with arrays and objects. Accept JSON via STDIN like cache-set), direct access to database can be handy way to tweak the system. Remember to clear caches after.