Separate merged SQL rows with a comma

Separate merged SQL rows with a comma

I'm having a bit of trouble getting my retrieved values from an SQL query
into the correct format.
I've managed to join multiple rows into the one value, however I am not
sure how to make it separate each of the values with a comma. Essentially
I need all the ID's of a product to be retrieved as, for example, if the
database had values of '5,6,9,1' '1,3,4' and '2,1' I want it to throw a
comma in between each like -> '5,6,9,1,1,3,4,2,1' instead is doing
something more like -> '5,6,911,3,42,1' which is what it is doing at the
moment.
The code I'm using is below. Any help would be greatly appreciated.
Thanks!
$hist = "SELECT ORDITEMS FROM cust_orderc WHERE ORDDATE >
to_date('".$olddate."','dd/mm/yyyy')";
$histitem = OCIParse($db, $hist);
OCIExecute($histitem);
while($row = oci_fetch_array($histitem)){
$pastitem .= $row['ORDITEMS'];
}
echo "$pastitem";