ช่วยด้วยครับ php sql error ทำไม่เป็นครับ งงมาก (ขอบคุณล่วงหน้าครับ)

<?php
session_start();
mysql_connect("localhost","root","123");
mysql_select_db("book");
mysql_query("SET NAMES UTF8");
date_default_timezone_set('Asia/Bangkok');
$Total = 0;
$datetime=date("Y-m-d H:i:s");
$a .= "'" . $datetime . "',";
$a .= "'" . $_SESSION["userid"] . "'";
$strSQL = "insert into order(order_date,user_id)values($a)";
mysql_query($strSQL) or die(mysql_error() . "<br />" . $strSQL);
$strorder_id = mysql_insert_id();
for($i=0;$i<=(int)$_SESSION["intLine"];$i++)
{
if($_SESSION["strpro_id"][$i] != "")
{
$strSQL = "insert into order_de(order_id,pro_id,order_de_qty,order_de_pirce)
VALUES
('".$strorder_id."','".$_SESSION["strpro_id"][$i]."','".$_SESSION["strorder_de_qty"][$i]."','".$SESSION["pro_pirce"][$i]."')
";
mysql_query($strSQL) or die(mysql_error());
}
}
mysql_close();
session_destroy();
header("location:finish_order.php?order_id=".$strorder_id);
?>