php พอกดปุ่ม back กลับไปแล้วข้อมูลที่ drop-down box หายไป อยากให้มันคงอยู่ไว้ ช่วยด้วยนะคะ

<option value="7" <?if($_SESSION["xyz"]=="7"){?>selected<?}?>>777</option>

<form name="frmMain" method="post" action="Sample2.php">
  Name 
  <input type="text" name="textfield">
  <br>
  Surname 
  <input type="text" name="textfield2">
  <br>
  Email 
  <input type="text" name="textfield3">
  <br>
  <input name="btnSubmit" type="submit" id="btnSubmit" value="Submit" onClick="clsControlObject('frmMain','write')">
  <input name="btnClear" type="button" id="btnClear" value="Clear" onClick="clsControlObject('frmMain','clear')">
</form>


 <script language="JavaScript">

function setCookie(name, value,	expires, path, domain, secure) {
    document.cookie= name + "="	+ escape(value)	+
	((expires) ? ";	expires=" + expires.toGMTString() : "")	+
	((path)	? "; path=" + path : "") +
	((domain) ? "; domain="	+ domain : "") +
	((secure) ? "; secure" : "");
}

function getCookie(name) {
    var	dc = document.cookie;
    var	prefix = name +	"=";
    var	begin =	dc.indexOf("; "	+ prefix);
    if (begin == -1) {
	begin =	dc.indexOf(prefix);
	if (begin != 0)	return null;
    } else {
	begin += 2;
    }
    var	end = document.cookie.indexOf(";", begin);
    if (end == -1) {
	end = dc.length;
    }
    return unescape(dc.substring(begin + prefix.length,	end));
}

function deleteCookie(name, path, domain) {
    if (getCookie(name)) {
	document.cookie	= name + "=" +
	    ((path) ? "; path="	+ path : "") +
	    ((domain) ?	"; domain=" + domain : "") +
	    "; expires=Thu, 01-Jan-70 00:00:01 GMT";
    }
}
</script>

<script language="JavaScript">
function clsControlObject(objForm,flgMode) 
{ 


  var form, ele_text,ele_hidden,ele_textarea,ele_hidden,ele_select,ele_hidden, i, elm; 
  form = document.getElementById 
    ? document.getElementById(objForm) 
    : document.forms[objForm]; 
    var valtext,valtextarea,valselect,valhidden
	valtext = "";
	valtextarea = "";
	valselect = "";
	valhidden = ""
								
	if (document.getElementsByTagName)
	{
			
			
			'******************* Input type=text *******************'
			ele_text = form.getElementsByTagName('input');
			for( i=0, elm; elm=ele_text.item(i++); )
			{
				
				if (elm.getAttribute('type') == "text")
				{
					
								if(flgMode=="write")
								{
									if(elm.value != "")
									{
									valtext=valtext.concat(elm.name+'||'+elm.value)+'(|)'
									}
								}
								else if(flgMode=="clear")
								{
									elm.value = ""							
									deleteCookie('cktext')
								}
								else if(flgMode=="read")
								{		
										if(getCookie('cktext') != null)
										{				
											splitString=getCookie('cktext').split("(|)")
											for(j=0;j<=parseInt(splitString.length-1);j++)
											{
												splitStringValue=splitString[j].split("||")
												if(elm.name == splitStringValue[0])
												{
													if(splitStringValue[1] != "")
													{
														elm.value = splitStringValue[1];
													}
													break;
												}
											}
										}
								}

					
				}
					
			}
			
			if(flgMode=="write")
			{			
				
				if(valtext != "")
				{
					setCookie('cktext',valtext)
				}
				
			}		
			'*****************  end Input type=text *****************'
			
			
			'******************* Input type=hidden *******************'
			ele_hidden = form.getElementsByTagName('input');
			for( i=0, elm; elm=ele_hidden.item(i++); )
			{
				
				if (elm.getAttribute('type') == "hidden")
				{
					
								if(flgMode=="write")
								{
									
									if(elm.value != "")
									{
									valhidden=valhidden.concat(elm.name+'||'+elm.value)+'(|)'
									}
								}
								else if(flgMode=="clear")
								{
									elm.value = ""							
									deleteCookie('ckhidden')
								}
								else if(flgMode=="read")
								{		
										if(getCookie('ckhidden') != null)
										{				
											splitString=getCookie('ckhidden').split("(|)")
											for(j=0;j<=parseInt(splitString.length-1);j++)

											{
												splitStringValue=splitString[j].split("||")
												if(elm.name == splitStringValue[0])
												{
													if(splitStringValue[1] != "")
													{
														elm.value = splitStringValue[1];
													}
													break;
												}
											}
										}
								}

					
				}
					
			}
			
			if(flgMode=="write")
			{			
				
				if(valhidden != "")
				{
					setCookie('ckhidden',valhidden)
				}
				
			}		
			'*****************  end Input type=hidden *****************'			
			
			
			
			
			
			
			'******************* Input type=textarea *******************'
			ele_text = form.getElementsByTagName('textarea');
			for( i=0, elm; elm=ele_text.item(i++); )
			{
				
				if (elm.getAttribute('type') == "textarea")
				{
					
								if(flgMode=="write")
								{
									if(elm.value != "")
									{
									valtextarea=valtextarea.concat(elm.name+'||'+elm.value)+'(|)'
									}
								}
								else if(flgMode=="clear")
								{
									elm.value = ""							
									deleteCookie('cktextarea')
								}
								else if(flgMode=="read")
								{		
										if(getCookie('cktextarea') != null)
										{				
											splitString=getCookie('cktextarea').split("(|)")
											for(j=0;j<=parseInt(splitString.length-1);j++)
											{
												splitStringValue=splitString[j].split("||")
												if(elm.name == splitStringValue[0])
												{
													if(splitStringValue[1] != "")
													{
														elm.value = splitStringValue[1];
													}
													break;
												}
											}
										}
								}

					
				}
					
			}
			
			if(flgMode=="write")
			{			
				
				if(valtextarea != "")
				{
					setCookie('cktextarea',valtextarea)
				}
				
			}		
			'*****************  end Input type=textarea *****************'
			
			
			
			'******************* Input type=select *******************'
			ele_text = form.getElementsByTagName('select');
			for( i=0, elm; elm=ele_text.item(i++); )
			{
				
				if (elm.getAttribute('type') == "select-one")
				{
					
								if(flgMode=="write")
								{
									if(elm.value != "")
									{
									valselect=valselect.concat(elm.name+'||'+elm.value)+'(|)'
									}
								}
								else if(flgMode=="clear")
								{
									elm.value = ""							
									deleteCookie('ckselect')
								}
								else if(flgMode=="read")
								{		
										if(getCookie('ckselect') != null)
										{				
											splitString=getCookie('ckselect').split("(|)")
											for(j=0;j<=parseInt(splitString.length-1);j++)
											{
												splitStringValue=splitString[j].split("||")
												if(elm.name == splitStringValue[0])
												{
																											
														if(splitStringValue[1] != "")
														{
															for (x=0;x<elm.length;x++)
															{
																	if (elm.options[x].value==splitStringValue[1])
																	{
																		elm.options[x].selected = true;
																		break;
																	}
															}	
														}												
														
													break;
												}
											}
										}
								}

					
				}
					
			}
			
			if(flgMode=="write")
			{			
				
				if(valselect != "")
				{
					setCookie('ckselect',valselect)
				}
				
			}		
			'*****************  end Input type=select *****************'			
														
		
		
	}
	
}

clsControlObject('frmMain','read');
</script>

<?php 
$Borrow_Id = $_POST['Borrow_Id'];
$B_Id = $_POST['B_Id'];
$Detail = $_POST['Detail'];
$Borrowdate = $_POST['Borrowdate'];
$Returndate = $_POST['Returndate'];
$Note = $_POST['Note'];
$product = $_POST['Product'];

$product_new =array_unique($product);

if(count($product_new)!=count($product))
{
echo "หมายเลข EQCode ที่เลือกมีรายการซ้ำกันอยู่กรุณากลับไปตรวจสอบด้วยนะคะ";
echo "<a href=\"javascript:history.back();\">กลับไปแก้ไขรายการ</a>";
exit();
}

// บันทึกไว้ตรงนี้ และไปอ่านเอาในส่วนของ javascript ในเพจก่อนหน้า
setcookie ('products', implode(',', $product));

$sql = "INSERT INTO stock(Borrow_Id,B_Id,Detail,Borrowdate,Returndate,Note) VALUES('$Borrow_Id','$B_Id','$Detail','$Borrowdate','$Returndate','$Note')";
//echo $sql;

if(mysql_query($sql))
{ 

echo "บันทึกข้อมูลเรียบร้อยแล้วคะ<br/>";
echo "<a href=\"borrow.php\">Back</a>";
} else {
	echo "ไม่สามารถบันทึกข้อมูลได้กรุณาลองใหม่<br/>";
	echo "<a href=\"javascript:history.back();\">Back</a>";
	}
	
	$result = count($product);
$i = 0;
while ( $i<$result ){
//echo $n;  
$sql="INSERT INTO borrow_product(Borrow_Id,EQCode) VALUES('$Borrow_Id','$product[$i]')";
//echo $sql."<hr>";
mysql_query($sql);
$sql = "UPDATE product SET status='ไม่ว่าง' WHERE EQCode='$product[$i]'";
//echo $sql."<hr>";
mysql_query($sql);
$i++;
}
?>

<?php 
$wantcount = true;
$counter_file = "lib/counter.txt";//สร้างไฟล์ใน notepad แล้วตั้งชื่อนี้ counter.txt แล้วpermission 777
if($wantcount){
if (file_exists($counter_file) and is_writeable($counter_file)){
$fp = fopen($counter_file,"r+") or die("Read File Error !");
$count = fread($fp, filesize($counter_file));
fclose($fp);
$fp = fopen($counter_file,"w+") or die("Write File Error !");
$count +=1;
$count =$count;
fputs($fp, $count);
fclose($fp);
}
}
?>