I made two changes and the error changed :

change 1

$sord = $_GET['sord'];

INTO

$sord = 'invid';

change 2

$SQL = “SELECT invid, invdate, amount, tax,total, note FROM invheader ORDER BY $sidx $sord LIMIT $start , $limit”;

INTO

$SQL = “SELECT invid, invdate, amount, tax,total, note FROM invheader ORDER BY $sidx , $sord LIMIT $start , $limit”;

The XML error now be this :

XML Parsing Error: junk after document element
Location: http://localhost/SpDatabase/res/grid/
Line Number 62, Column 1:
11112001-01-10103.0045.00149.00
^