assigning variable a value based on select box

[pjc2003]

hi ive created a page that allows a user to select a value for variable "$location_name" from a select box, I wish to put some code in that assigns a value to another variable "$location_id" based on whatever value is currently given to "$location_name"..

Is it possible to do it with an if or while loop?

here is my code so far:

<?php
require('./header.php');
require('./dbconn.php');

//Gets the variables part number and location id from previous page where they were POSTED...
$part_number = $_GET['part_number'];

$action = 'show_form';
if(!empty($_POST)) {
$action= 'process_data';

}


if($action== 'process_data')  {

$part_number = $_POST['part_number'];
$location_id = trim($_POST['location_id']);      ------- how can i get this to equal number based on what location_name equals??       
$location_name = trim($_POST['location']);
$valid = intval(trim($_POST['valid']));

$sql= "UPDATE stockdata set location = '$location_name', location_id = '$location_id', valid = '$valid' WHERE  part_number= '$part_number'  ";
$ok= mysql_query($sql) or die(mysql_error());

if($ok){
echo "<p class='ErrorText'>Update OK!</p>";

}
else
{
echo "<p class ='ErrorText'>Update Failed!</p>";
}
}
else {
$action = 'show_form';
}

echo $sql;   

//end running database query


if($action == 'show_form')  {
?>



<form method="post" action="stock_control.php">

<input type="hidden" name="part_number" value="<?=$part_number?>" />
<input type="hidden" name="location_id" value="<?=$location_id?>" />


  <p class ='MainText'>
  Change Location -
      <select name="location">
      <option =1 <?php if ($location_name ="A") echo ' selected '?> >A</option>
      <option =2 <?php if ($location_name ="B") echo ' selected '?> >B</option>
  <option =3 <?php if ($location_name ="C") echo ' selected '?> >C</option>
      <option =4 <?php if ($location_name ="D") echo ' selected '?> >D</option>
  <option =5 <?php if ($location_name ="E") echo ' selected '?> >E</option>
      <option =6 <?php if ($location_name ="F") echo ' selected '?> >F</option>
  <option =7 <?php if ($location_name ="G") echo ' selected '?> >G</option>
      <option =8 <?php if ($location_name ="H") echo ' selected '?> >H</option>
  <option =9 <?php if ($location_name ="I") echo ' selected '?> >I</option>
      <option =10 <?php if ($location_name ="J") echo ' selected '?> >J</option>
  <option =11 <?php if ($location_name ="K") echo ' selected '?> >K</option>
      <option =12 <?php if ($location_name ="L") echo ' selected '?> >L</option>
  <option =13 <?php if ($location_name ="M") echo ' selected '?> >M</option>
      <option =14 <?php if ($location_name ="N") echo ' selected '?> >N</option>
  <option =15 <?php if ($location_name ="O") echo ' selected '?> >O</option>
      <option =16 <?php if ($location_name ="P") echo ' selected '?> >P</option>
  <option =17 <?php if ($location_name ="Q") echo ' selected '?> >Q</option>
      <option =18 <?php if ($location_name ="R") echo ' selected '?> >R</option>
  <option =19 <?php if ($location_name ="S") echo ' selected '?> >S</option>
      <option =20 <?php if ($location_name ="T") echo ' selected '?> >T</option>
  <option =21 <?php if ($location_name ="U") echo ' selected '?> >U</option>
      <option =22 <?php if ($location_name ="V") echo ' selected '?> >V</option>
  <option =23 <?php if ($location_name ="W") echo ' selected '?> >W</option>
      <option =24 <?php if ($location_name ="X") echo ' selected '?> >X</option>
  <option =25 <?php if ($location_name ="Y") echo ' selected '?> >Y</option>
      <option =26 <?php if ($location_name ="Z") echo ' selected '?> >Z</option>
    </select>
</p>
  <p class ='MainText'>
    Change Validity-
      <select name="valid">
      <option =1 <?php if ($valid =0) echo ' selected '?> > 0</option>
      <option =2 <?php if ($valid =1) echo ' selected '?> > 1</option>
    </select>
</p>
  <input type="submit" value="save"/>
  </p>
</form> 
<?php
}
require('./footer.php');
?>


many thanks,

pete.