function _create_mysql_update_string($source_array, $string_fields = array(), $numeric_fields = array())
// $source_array is an array such as $_REQUEST or $_POST.
// $string_fields
// is a list of keys such that $source_array[$key] should be a string.
// $numeric_fields
// is a list of keys such that $source_array[$key] should be a numeric value.
// This function checks that those expectations are satisfied, and constructs a
// string of column=value 'assignments' that can be used in an SQL UPDATE