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Edit phpMyAdmin's config.inc.php and

add or update LoginCookieValidity the value as follows:

$cfg['LoginCookieValidity'] = 3600 * 9; // 9 hours

Update: (2009-12-07)

If the variable above doesn't work for you maybe you can do this

<code>/* Authentication type */
$cfg['Servers'][$i]['auth_type'] = 'config';
$cfg['Servers'][$i]['host'] = 'localhost';
$cfg['Servers'][$i]['user'] = 'root'; 
$cfg['Servers'][$i]['password'] = 'your-root-pwd';
</code>

Note/Warning/Security Concern:

Do this only on your local/development machine and not on production server !!!

REFERENCE
http://www.devcha.com/2008/01/extend-phpmyadmin-session-timeout-value.html