Form problem: Getting info from one table to insert into another table

[singingsands]

I have a form to submit information about weekend retreats to a database, one of which is location. I have a lookup table for locations so the user can select from a list.  The Locations table has a Primary Key called Location_ID and the Weekends table has a Foreign Key called Location_ID_FK.

I'm having trouble saving the Location_ID (from a query) to Location_ID_FK.  This is the query to select the location:

Quote

<div class="form-group">
       <label for="Location" class="col-sm-4 control-label">Location</label>
        <div class="col-sm-10">
             <select name="std" class="form-control col-sm-8">  
             <?php   
                   $query = "SELECT * FROM locations";
                    if ($result = $conn->query($query)) {  
                      while ($row = $result->fetch_assoc()) {
                      echo "<option value=".$row["Location_ID"].">".$row["Location_Name"]."</option>";
                       } 
                    }
               ?>
               </select>
           </div>  
 </div> 

This in itself works, I get the list of locations.
But when I try to submit the form  I keep getting the message "Warning: Undefined array key "Location_ID"..."
I've tried everything I can think of, including changing the name of the field Location_ID_FK to Location_ID, I've also tried putting this line after the 'while' statement.
 

Quote

 $Location_IK_FK = $row["Location_ID"];

I've also tried adding this same line in the section defining variables to be inserted.

I should add that when I use a simple input text for Location the insert query works, and I've triple-checked that I'm using the correct variable in the insert query.

What am I missing?  Any help is gratefully appreciated.

Edited November 29 by singingsands
add more information

[maxxd]

PHP uses the form element's name attribute to name the _POST or _GET variable. Your select element is named 'std', not 'Location_ID'.

[singingsands]

I can't believe I missed something so basic! My face is red.

Thank you!

[maxxd]

Happens to the best of us!