So far I know that $this-> is used in object programming to access methods and properties of current object. But why $this-> is not used also for $q in the following code. When exactly do we need to put $this->?

class MySQLDB
{
function usernameTaken($username){
if(!get_magic_quotes_gpc()){
$username = addslashes($username);
}
$q = "SELECT username FROM ".TBL_USERS." WHERE username = '$username'";
$result = mysql_query($q, $this->connection);
return (mysql_numrows($result) > 0);