you need mod_rewrite module

in the .htaccess file,

Options +FollowSymLinks
RewriteEngine on
RewriteCond %{HTTP_USER_AGENT} ^whateveragent1.* [OR]
RewriteCond %{HTTP_USER_AGENT} ^whateveragent3.* [OR]
...
RewriteCond %{HTTP_USER_AGENT} ^whateveragentn.*
RewriteRule ^/index.php http://example.mobi/otherurl.php [R=301,L]

That should do it.