How can I modify the values in variable table using phpmyadmin? I modify the site name in variable table using phpmyadmin as below.

s:21:"example@example.com";  to  s:21:"example@example.info"; 

But then when I run cron I get the follwoing message.


  Warning: Cannot modify header information - headers already sent by (output started at /public_html/includes/bootstrap.inc:566) in /public_html/includes/bootstrap.inc on line 739


This is just a examle, but if I change something in variable table using phpmyadmin I always get the same error.

When I modify variable table do I need to modify also other tables at the same time ?
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.