mysql - PHP Drop Down Selected -

i've got 3 drop down boxes populate each drop down 1 above on 3rd drop down want bring value selected. code below

<tr> <td width="119">category</td>  <td width="371">    <select name="cat_id" id="cat_id" onchange="showcompany(this.value);">    <option value="">--select--</option>    <?php     $sql="select * category";    $sql_row=mysql_query($sql);    while($sql_res=mysql_fetch_assoc($sql_row))    {    ?>    <option value="<?php echo $sql_res["id"]; ?>" <?php if($sql_res["id"]   ==$_request     ["cat_id"]) { echo "selected"; } ?>><?php echo $sql_res["category_name"]; ?  ></option>     <?php     }    ?>    </select>    </td>  </tr>   <tr> <td>company</td> <td id="td_company">    <select name="company_id" id="company_id">;    <option value="">--select--</option>    <?php    $sql="select * company cat_id='$_request[cat_id]'";    $sql_row=mysql_query($sql);    while($sql_res=mysql_fetch_assoc($sql_row))    {    ?>    <option value="<?php echo $sql_res["id"]; ?>"><?php echo $sql_res["company_name"]; ?></option>    <?php    }    ?>  </select>    </td>  </tr>  <tr>  <td>item</td> <td id="td_item">    <select name="item_id" id="item_id">    <option value="">--select--</option>    <?php    $sql="select * item comp_id='$_request[cat_id]'";    $sql_row=mysql_query($sql);    while($sql_res=mysql_fetch_assoc($sql_row))    {    ?>    <option value="<?php echo $sql_res["id"]; ?>"><?php echo $sql_res["item_name"]; ?></option>    <?php    }    ?> </select>    </td>  </tr>  <tr> <td>price</td> <td id="td_item">    <?php    $sql="select unit_cost item comp_id='$_request[cat_id]'";    $sql_row=mysql_query($sql);    while($sql_res=mysql_fetch_assoc($sql_row))    {    echo "<td>" . "&pound" . $sql_res['unit_cost'] . "</td>";    }    ?>     </td>  </tr>   <tr>  </tr> </table> </form> 

so want bring price option selected, @ moment bringing prices.

e.g. first drop down select cellings company drop down select access panels 3rd drop down 3 items (arm strong, track , screw) , brings 3 prices want able select 1 , bring price. ideas?

first off shouldn't using mysql_* functions, they're deprecated. either mysqli or pdo. second, have vulnerability in code inserting $_request straight sql. that's called sql injection. should reading on it.

with in mind problem query trying select of items company id = category id. query:

$sql="select * item comp_id='$_request[cat_id]'"; 

notice comp_id='$_request[cat_id]' should instead comp_id = $_request[comp_id]'?