/////////////////////////////////////////////////////////////the function

    public function query_insert($sqlline){
        $this->sqlline=$sqlline; // etc INSERT INTO table_name (column1, column2, column3,...) VALUES (value1, value2, value3,...)
        mysql_insert($this->sqlline) or die("failure in insert : " . mysql_error());
    }

///////////////////////////////////////////////////////////the test file
include 'class/sql.class.php'; // includes the sql.class

$usrusername = $_POST['usrusername'];
$usrpassword = $_POST['usrpassword'];
$usrpassword = md5($this);

echo "<p>selected user : " . $usrusername . "</p>" . "<p>selected password : " . $usrpassword . "</p>";


 $sqlfunction = new sqldo(); // Calls sqldo object in sql.class

    $statement = "INSERT INTO sl_users (id, username, password) VALUES ('', '$usrusername', '$usrpassword')";
    $sqlfunction->query_insert($statement);