สอบถามเรื่องทำ login session ของ php + mysql หน่อยคับ งง มากก

@ob_start();
@session_start();
include("../lib/utility.php");
connect_db();

$u = $_REQUEST["u"];
						$p = sha1($_REQUEST["p"]);
						$ip =  $_SERVER["REMOTE_ADDR"];
						
						if(isset($u)){
							if(!isset($u) and !isset($p)){
								echo "<img src='image/warned.gif'><font color='#FF0000'>คุณไม่ได้กรอกชื่อผู้ใช้ หรือรหัสผ่าน</font>";
							}else{
								$sql_chk1 = "select count(user_id) as cou from tb_xuserx where user_user = '$u'";
								$rel_chk1 = mysql_query($sql_chk1) or die(mysql_error());
								$row_chk1 = @mysql_fetch_array($rel_chk1);
								$chk_u = $row_chk1["cou"];
								if($chk_u == 0){
									echo "<img src='image/warned.gif' ><font color='#FFCC00'>ไม่มีชื่อผู้ใช้นี้ในระบบ..กรุณาอย่าพยายามเข้าใช้งานระบบ</font>";
								}else{
									$sql_chk2 = "select * from tb_xuserx where user_user = '$u'";
									$rel_chk2 = mysql_query($sql_chk2) or die(mysql_error());
									$row_chk2 = @mysql_fetch_array($rel_chk2);
									$u_in = $row_chk2["user_user"];
									$p_in = $row_chk2["user_pass"];
									$level_in = $row_chk2["user_level"];
									$id_ = $row_chk2["user_id"];
									
									if($u_in != $u and $p_in != $p){
										echo "<img src='image/warned.gif' ><font color='#FFCC00'>ชื่อผู้ใช้หรือรหัสผ่านไม่ถูกต้อง..กรุณาลองอีกครั้ง</font>";
									}else{
										echo "1 : ".$_SESSION["name_full_sup"] = $row_chk2["user_name"]." ".$row_chk2["user_surname"];
										echo "2 : ".$_SESSION["username_in_sup"] = $u_in;
										echo "3 : ".$_SESSION["level_sup"] = $level_in;
										echo "4 : ".$_SESSION["id_sup"] = $id_;
										
										if($_SESSION["username_in_sup"] != ""){
											echo"<script>window.location='dashboard.php';</script>";
										}else{
											echo "<img src='image/warned.gif' ><font color='#FFCC00'>กรุณาล็อกอินใหม่..</font>";
										}
									}
								}
							}
						}

@ob_start(); 
@session_start();

include("../lib/utility.php");
connect_db();
check_login("name_full_sup","index.php");


echo "1 : ".$_SESSION["name_full_sup"];
echo "2 : ".$_SESSION["username_in_sup"];
echo "3 : ".$_SESSION["level_sup"];
echo "4 : ".$_SESSION["id_sup"];