Hi fellow developers [I'm a newby]

I have researched my problem for days now without success ... I must be missing something simple ... please help :

RSFORM Pro Form name: Stolen_Bikes Form Id : 15
Dropdown Field Name : Stolen_Make [lists the User specific items which has been populated via MySQL query in Group_Concat format. The User chooses the item that needs actioning]
Hidden Field : Stolen_SubmissionId [this field will be populated by MySQL query or JS based on the Dropdown selection]

I want to use the hidden field value [Stolen_SubmissionId] in a MySQL query [UPDATE a specific record in database after form submission] within the $_POST script tag .

I have tested the following code which works when I substitute the $Stolen_SubmissionId with value but does not work when I use a Variable reference [in red below]... it seems that the value in the hidden field is not made available to $_POST:

if(isset($_POST))
{
$Stolen_SubmissionId=$_POST;
$db =& JFactory::getDBO();
$db->setQuery("UPDATE jos_rsform_submission_values SET FieldValue='Yes'
WHERE jos_rsform_submission_values.SubmissionId='$Stolen_SubmissionId';
AND FieldName='Bike_StolenConfirm'");
$db->query();
}

What am I doing wrong?

William.