Call my php script from a javascript function

I have a form that upon submit calls a php script to create a drop down list based on a database query. I then need to be able to select an item in the dropdown list, hit a button and have a form populated with the results.

My form is set up like this: with one submit button, and another button type that calls a javascript function.

<form name="searchForm" action="newCustomerSearchform.php" method="post">
 <label><span></span> <input type="text" name="searchDB" /></label>
 <button type="submit" name="btnSearch" value="Search"  id="btnSearch" onclick="this.form.action">Search</button></label>

<?php
  if (isset($_SESSION['names'])){
        echo  '<select  id ="customerDD" name="customerDD" class="customers">';

        foreach ($_SESSION['names'] as $option => $value) {
                echo  '<option value='.$value['ID'].'>'.$value['First_Name'].' '.$value['Surname'].'</option>';
}
        echo  '</select>';
}
if (isset($_SESSION['addnames'])){
        echo 'set';
       echo $_SESSION['addnames'];
  }
?>
 <button type="button" name="btnAdd" value="add" id="btnAdd" onclick=addcustomer();> Add</button>
</form>

The second button calls:

<script type="text/javascript">
var temp = null;
function addcustomer()
{
temp = $("#customerDD").val();
$.post('newCustomerAdd.php', {variable: temp});
}
</script>

When I test this using

document.write(test);

it displayed the correct value of what I curentlly had selected in the drop down.

newCustomerAdd.pp contains a query to my database using the variable from the javascript function and then saves this in a session variable which i use to display on the main page.

<?php
include 'newCustomer.php';
connect('final');

    $temp = $_POST['temp'];
    $temp = htmlspecialchars($temp); // stop HTML charkacters
    $temp= mysql_real_escape_string($temp); //stop SQL injection
    $query = "SELECT * FROM customer WHERE ID = '$temp";
    $data = mysql_query($query) or die(mysql_error());


    $Customers = array();
    $colNames = array();
    while($row = mysql_fetch_assoc($data)){// puts data from database into array, loops until no more

    $Customers[] = $row;

        }
         $anymatches = mysql_num_rows($data); //checks if the querys returned any results
        if ($anymatches != 0) {
        $_SESSION['addnames']=$Customers;

    $colNames = array_keys(reset($Customers));




    }

            if ($anymatches == 0) 
                    { 
                        echo "Sorry, but we can not find an entry to match your query<br><br>"; 
                    } 




header("location: newCustomer.php");
?>

I then test if this variable is set in the form and echo the variable, but nothing is being produced.

It seems like there is something wrong with my Jquery, as the variable is not getting posted to the php script.