PHP: How to determine if a database connection is open

I have the following class method -

class Someclass
{
    /* Other properties and methods */

    public function close_connection($connection=NULL)
    {
        return mysql_close($connection)
    }
}

Now, before calling the mysql_close function in the above code, I want to check if the $connection points to a open database connection. That is, I want to be sure that the connection I am closing is open and has not already been closed.

How can I do that?