เคยเจอปัญหาแบบนี้ไหมครับ ผมรับค่ามาจาก list/menu 2 ตัว คือ country กับ city โดย 2 ตัวนี้ใช้ ajax ตอนรับค่ามาก็ $_POST["country"]

 <script type="text/javascript">
		 
		 function Inint_AJAX() 
		{
           try { return new ActiveXObject("Msxml2.XMLHTTP");  } catch(e) {} //IE
           try { return new ActiveXObject("Microsoft.XMLHTTP"); } catch(e) {} //IE
           try { return new XMLHttpRequest(); } catch(e) {} //Native Javascript
           alert("XMLHttpRequest not supported");
           return null;
         };

       function dochange(src, val) 
	   {
        
		var req = Inint_AJAX();
        req.onreadystatechange = function () 
		{ 
          if (req.readyState==4) 
		   {
               if (req.status==200) 
			   {
                    document.getElementById(src).innerHTML=req.responseText; //ÃѺ¤èÒ¡ÅѺÁÒ
               } 
          }
        };
     
	    req.open("POST", "getcountry.php?data="+src+"&val="+val); //ÊÃéÒ§ connection
        req.setRequestHeader("Content-Type", "application/x-www-form-urlencoded;charset=tis-620"); // set Header
        req.send(null); //Ê觤èÒ
       }
window.onLoad=dochange('country', -1);

 </script>

// list/menu country
 echo "<font id='country'><select style='width: 120px; border: 1px solid #336699;'>\n";
 echo "<option value='0'>--------------------</option> \n" ;
 echo "</select></font>\n";

// list/menu city
 echo "<font id='city'><select style='width: 120px; border: 1px solid #336699;'>\n";
 echo "<option value='0'></option> \n" ;
 echo "</select></font>\n";

include_once("includes/config.inc.php");

        
     header("content-type: application/x-javascript; charset=tis-620");
     
     $data=$_GET['data'];
     $val=$_GET['val'];     
         
     if ($data=='country') {
          $sql="SELECT * FROM country_tbl ORDER BY  country_name, country_code";
          $result = mysql_query($sql);
          echo "<select name='country' id='country' style='width: 120px; border: 1px solid #336699;' onChange=\"dochange('city', this.value)\">\n";
          echo "<option value='0'>- Select -</option>\n";
         
          while($fetcharr = mysql_fetch_array($result)) 
		  { 
               $val = $fetcharr['country_code'];
               $label = $fetcharr['country_name'];
               echo "<option value=\"$val\">$label</option> \n" ;
          }
		  
     } else if ($data=='city') {
          echo "<select name='city' id='city' style='width: 120px; border: 1px solid #336699;' onChange=\"dochange('', this.value)\">\n";
          $sql="select * from city_tbl where country_code = '$val'";
          $result = mysql_query($sql);
          echo "<option value='0'>-- Select --</option>\n";
          
          while($fetcharr = mysql_fetch_array($result)) 
		  { 
               $val = $fetcharr['city_id'];
               $label = $fetcharr['city_name'];
               echo "<option value=\"$val\">$label</option> \n" ;
          }
     } 
 echo "</select>\n";

$country = $_POST["country"];
$city = $_POST["city"];