Thanks a lot. its working now. but after Adding Line Item we should

unset($_POST['_stock_id_edit']);

Because in new line item code and description is not displaying in correct combination. that is different.

@kvvaradha please see this

Item selection not working in purchase order and sales order entry for items with editable item description.

when we select a item "A" having code "001" with editable description and if we want to change this item with another item by clicking Item Search Button, We now select Item "B" having code "002" after that we click Add Line Item. then item code and item description are not same as we have selected item "B".