php - MySQLi UPDATE where clause not working -

i wrote code suposed update "user level" stored in column "level" in table "users", user specified variable "$x5_input_username".

$x_changelvl_sql = "update users set level='$x5_input_newlevel' `username`=$x5_input_username"; 

when using userlevel "2" , username "testuser" (which both exist in database), gives me error:

error updating record: unknown column 'testuser' in 'where clause'

quote string in where clause

$x_changelvl_sql = "update users      set level='$x5_input_newlevel'      `username`='$x5_input_username'"; 

better yet, start using mysqli prepared statements , bind variables