How Can I combine a list from XML in PHP -

i have parts list coming xml file, goes below. want able generate using php combined list. example "part 1" recorded twice. want qty show 13 under part 1 when generated either in json output or xml output. best way of doing that? looked @ php function array_combine wasn't able figure out if values combined mathematically instead of showing 1 of results. loading xml url in simplexml_load_file() function. thank help

my xml url:

   <db>       <record>          <part>part 1</part>          <qty>4</qty       </record>     <record>          <part>part 2</part>          <qty>5</qty     </record>     <record>          <part>part 1</part>          <qty>9</qty     </record>   </db> 

want display:

   <db>       <record>          <part>part 1</part>          <qty>13</qty>       </record>     <record>          <part>part 2</part>          <qty>5</qty     </record>   </db> 

iterate on xml document, accumulating part quantities go:

$simplexmlelement = new simplexmlelement(<<<xml <db>     <record>         <part>part 1</part>         <qty>4</qty>     </record>     <record>         <part>part 2</part>         <qty>5</qty>     </record>     <record>         <part>part 1</part>         <qty>9</qty>     </record> </db> xml );  $quantities = array(); foreach ($simplexmlelement $child) {     if ($child->part && $child->qty) {         $part = (string)$child->part;         if (!isset($quantities[$part])) {             $quantities[$part] = 0;         }         $quantities[$part] += (int)$child->qty;     } } echo json_encode($quantities);