This variable does not seem to be used.

libraries/display_tbl.lib.php

@@ -717,10 +717,6 @@ function PMA_displayTableHeaders(&$is_display, &$fields_meta, $fields_cnt = 0, $
         $GLOBALS['mime_map'] = PMA_getMIME($db, $table);
     }
 
-    if ($is_display['sort_lnk'] == '1') {
-        $select_expr = $analyzed_sql[0]['select_expr_clause'];
-    }
-
     // See if we have to highlight any header fields of a WHERE query.
     // Uses SQL-Parser results.
     $highlight_columns = array();