สอบถามครับ ทำไมเวลา กดSubmit เเล้ว $_SESSION['name']; หาย ตรอดเลยครับ

<?
session_start();
include('include/connectdb.php');
include ("function.php");
$id_user=$_SESSION['name'];
if($_POST['OK']){
$sql=("insert into personal_data (id_user) values ('$id_user') "); 
mysql_query($sql,$link);
$sql=("insert into contract_information (id_user) values ('$id_user') "); 
mysql_query($sql,$link);
$sql=("insert into regis_1 (id_user) values ('$id_user') "); 
mysql_query($sql,$link);
$sql=("insert into education_background (id_user) values ('$id_user') "); 
mysql_query($sql,$link);
$sql=("insert into work_experience (id_user) values ('$id_user') "); 
mysql_query($sql,$link);
$sql=("insert into special_skill (id_user) values ('$id_user') "); 
mysql_query($sql,$link);
echo "<script>alert('กรุณาใส่ข้อมูลให้ครบทุกหมวด \\n\\n');window.location='register.php?st=2';</script>";
}

if($_POST['regis_']){
$id_user=$_SESSION['name'];
$name_t=$_POST['name_t'];
$nickname_t=$_POST['nickname_t'];
$name_e=$_POST['name_e'];
$nickname_e=$_POST['nickname_e'];
$Employee=$_POST['Employee'];
$gubl=$_POST['gubl'];
$Position_1=$_POST['Position_1'];
$Position_2=$_POST['Position_2'];
$Expected_Salary=$_POST['Expected_Salary'];
$wan=$_POST['wan'];
$Available=$_POST['Available'];
if($Available!="ทันที"){
	$Available=$_POST['wan'];
	}else{
	$Available="ทันที";
	}
			$tmp_name = $_FILES["file"]["tmp_name"];
        	$name= $_FILES["file"]["name"];
        	$size = $_FILES["file"]["size"];
        	$type = $_FILES["file"]["type"];
			if($type=="image/gif")
				{
					$name_m=date("YmdHis$key").".gif";
				}
				else
				{
					$name_m=date("YmdHis$key").".jpg";
				}
				$folder= "../image/";	
			move_uploaded_file($tmp_name,"$folder$name_m");	

$sql=("Update regis_1 set 
name_t='$name_t',
nickname_t='$nickname_t',
name_e='$name_e',
nickname_e='$nickname_e',
employee='$Employee',
gubl='$gubl',
position_1='$Position_1',
position_2='$Position_2',
expected_salary='$Expected_Salary',
available='$Available',
file_='$name_m',
wan='$wan'
where id_user='$id_user' "); 
mysql_query($sql,$link);
echo "<script>alert('บันทึกข้อมูลเรียบร้อยเเล้ว \\n\\n');window.location='register.php?st=2';</script>";
}
?>