PHP เพิ่มช่องค้นหาแบบ Dropdown list menu เข้าไปค้นหาให้ย่อยลงไปอีกจะใช้คำสั่งอะไรดีครับ

<form name="form1" method="get" action="<?=$_SERVER['SCRIPT_NAME'];?>">
  <table width="1100" border="1" align="center">
    <tr>
      <th>
  
 Room <input name="room" type="text" id="room" value="<?=$_GET["room"];?>">
 Row <input name="row" type="text" id="row" value="<?=$_GET["row"];?>">
 Rack <input name="rack" type="text" id="rack" value="<?=$_GET["rack"];?>">  	Detail <input name="storage" type="text" id="storage" value="<?=$_GET["storage"];?>">   
 
      <select name="select">
      	<option value="" selected="selected">--- Select ---</option>
        <option value="sourcelocation">Source Location</option>
        <option value="sourceport">Source Port</option>
        <option value="interface">Interface</option>
        <option value="protocol">Protocol</option>
        <option value="product">Product</option>
        <option value="devicename">Devicename</option>
        <option value="ipaddress">IP Address</option>
        <option value="destinationport">Destinationport</option>
        <option value="destinationlocation">Destinationlocation</option>
        <option value="remark">Remark</option>
    </select>  
        
  keyword <input name="storage" type="text" id="storage" value="<?=$_GET["keyword"];?>">   
        
		<input type="submit" name="submit" value="Search"></th>

  </table>