changez-le comme ceci pour le séparer par une virgule.
$_query = "INSERT INTO ord_dets(Order_ID,custmer_ip,Resturant_ID,Resturant_name,City_name,
Product_Id,Product_Name,Product_Price,item_sub)
VALUES ";
$_query_parts = array();
$sep ="";
for($x=0;$x<count($OI); $x++){
$_query_parts = "$sep('" . $OI[$x] . "', '" . $ip[$x] . "','" . $_SESSION['rest_id'][$x] . "','" . $rest_name[$x] . "',
'" . $City_name[$x] . "','" . $Product_Id[$x] . "','" . $product_name[$x] . "','" . $prod_price[$x] . "','" . $item_sub[$x] . "')";
$sep=",";
}
$query_run = $dbc->prepare($_query);
echo $_query;
if (!$query_run->execute()) {
$insertError = "There was an error inserting data: " . $query_run->error;
print "affected rows:" . $query_run->affected_rows; //how many records affected?
}
}