php - Unset element in array

I want to unset 1 element in the array.
If for example I use GET and ?group=k

How do I unset "k" in the array?

This is the array:

$groups_array = array('a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','a2','b2','c2','d2','e2','f2');

I have tried

   if(isset($_GET['group'])) {
     unset($groups_array[1]);
     $new_groupps_array = array_values($groups_array);
   }    

which works fine but where it shows [1] it needs to be a letter so I know how to unset it?

Hope you understand
many thanks