array(4) {
  [0]=>
  array(2) {
    ["id"]=>
    string(4) "id-1"
    ["articles"]=>
    array(2) {
      [0]=>
      array(1) {
        ["data"]=>
        array(6) {
          ["art"]=>
          string(6) "108108"
          ["type"]=>
          string(0) ""
          ["pos"]=>
          string(3) "125"
          ["kleur"]=>
          string(4) "3750"
          ["height"]=>
          string(3) "179"
          ["foto"]=>
          string(2) "01"
        }
      }
      [1]=>
      array(1) {
        ["data"]=>
        array(6) {
          ["art"]=>
          string(0) ""
          ["type"]=>
          string(0) ""
          ["pos"]=>
          string(3) "163"
          ["kleur"]=>
          string(0) ""
          ["height"]=>
          string(0) ""
          ["foto"]=>
          string(0) ""
        }
      }
    }
  }
  [2]=>
  array(2) {
    ["id"]=>
    string(7) "ui-id-2"
    ["articles"]=>
    array(2) {
      [0]=>
      array(1) {
        ["data"]=>
        array(6) {
          ["art"]=>
          string(0) ""
          ["type"]=>
          string(0) ""
          ["pos"]=>
          string(3) "944"
          ["kleur"]=>
          string(0) ""
          ["height"]=>
          string(0) ""
          ["foto"]=>
          string(0) ""
        }
      }
      [1]=>
      array(1) {
        ["data"]=>
        array(6) {
          ["art"]=>
          string(0) ""
          ["type"]=>
          string(0) ""
          ["pos"]=>
          string(3) "586"
          ["kleur"]=>
          string(0) ""
          ["height"]=>
          string(0) ""
          ["foto"]=>
          string(0) ""
        }
      }
    }
  }
}

foreach($data as $key => $value) {
    if($value['id'] === "id-1") {
        foreach($value as $tweek => $tweev) {
            foreach($tweev as $driek => $driev) {
                foreach($driev as $vierk => $vierv) {
                    if($vierv['pos'] === "163") {
                        // This is the right article! I think...
                        echo $vierv['pos'] . "<br />"; // shows 163!
                        foreach($vierv as $vijfk => $vijfv) {
                            // What to do?!?!
                        }
                    }
                }
            }
        }
    }
}

<?php
     $a1=array("a"=>"red","pos"=>"364");
     $a2=array($a1,"1"=>"blue","2"=>"yellow");//one array inside another to simulate your multi depth array

     array_walk_recursive($a2,"myfunction");//use array_recursive


    function myfunction($value,$key)
    {
       if ($key == 'pos' && $value == '364') { //set your condition here
        echo "change your key or value here";
       }
    }
    ?>

arr = array(
  0=>array(
     'id'=>"id-1",
     'articles'=> array(
        0=>array(
           'data'=> array(
              'art'=> "108108"
              'type'=> ""
              'pos'=> "125"
              'kleur'=> "3750"
              'height'=> "179"
              'foto'=> "01"
              ),
           ),


        1=> array(
            'data'=> array(
               'art'=> ""
               'type'=> ""
               'pos'=> "163"
               'kleur'=> ""
               'height'=> ""
               'foto'=> ""
               ),
            ),
      ),
     ),
  2=> array(
    'id'=>"ui-id-2"
    'articles'=> array(
       0=> array(
           'data'=> array(
              'art'=> ""
              'type'=> ""
              'pos'=> "944"
              'kleur'=> ""
              'height'=> ""
              'foto'=> ""
              ),
           ),
       1=> array(
           'data'=> array(
              'art'=> ""
              'type'=> ""
              'pos'=> "586"
              'kleur'=> ""
              'height'=> ""
              'foto'=> ""
              ),
           ),
       ),
    ),
  );

foreach ($arr as $key=>value)
   if ($value['id'] == 'id-1')
      foreach($value as $key2=>$value2)
         if ($value2['pos'] == 163)
             $value2['art'] = $newArtValue;

$arr = array(
  'id-1'=>array(
     'articles'=> array(
        125=>array(
              'art'=> "108108"
              'type'=> ""
              'kleur'=> "3750"
              'height'=> "179"
              'foto'=> "01"
           ),
        163=> array(
               'art'=> ""
               'type'=> ""
               'kleur'=> ""
               'height'=> ""
               'foto'=> ""
            ),
      ),
     ),
  'ui-id-2'=> array(
    'articles'=> array(
       944=> array(
              'art'=> ""
              'type'=> ""
              'kleur'=> ""
              'height'=> ""
              'foto'=> ""
           ),
       586=> array(
              'art'=> ""
              'type'=> ""
              'kleur'=> ""
              'height'=> ""
              'foto'=> ""
           ),
       ),
    ),
  );

$arr['id-1'][163]['art'] = $newArtValue;