Not sure about that: don't you mean
$string="productInventory='042' OR productInventory='002";
$query="select * from storeProduct where $string order by productCategory, productName ASC"
And of course for an arbitrary number of productInventory fields:
$array=array('042','002','014');
foreach($array as $key=>$elt)
{ $array[$key]="productInventory='$elt'";
}
$string = implode(' OR ',$array);