Use get_display() method.

sample code:

$dg = new C_DataGrid(“SELECT * FROM orders”, “orderNumber”, “orders”);
$dg→display(true, false)
$mygrid = $dg→get_display(true);// true to include required javascript library

// display the grid later like this
echo $mygrid;

Online reference:
http://phpgrid.com/documentation/get_display/