Cuando lo hagoprint_r($nodes); Yo obtengo:

Array (
  [0] => SimpleXMLElement Object (
    [kategoria_6] => 1
    [opis_6] => 1
    [img] => slides/slide0.jpg
  )
  [1] => SimpleXMLElement Object (
    [kategoria_6] => 2
    [opis_6] => 2
    [img] => slides/slide1.jpg
  )
  [2] => SimpleXMLElement Object (
    [kategoria_6] => 3
    [opis_6] => 3
    [img] => slides/slide4.jpg
  )
  [3] => SimpleXMLElement Object (
    [kategoria_6] => 4
    [opis_6] => 4
    [img] => slides/slide3.jpg
  )
  [4] => SimpleXMLElement Object (
    [kategoria_6] => 7
    [opis_6] => 7
    [img] => slides/slide2.jpg
  )
)

Y ahora, en este caso, no puedo hacer esto:

$nodes->asXML("c:\\aaa.xml");

Porque esta es una matriz que podría hacer f.ex:

$nodes[1]->asXML("c:\\aaa.xml");

Pero solo obtendré un elemento. Necesito obtener todo este arreglo, o mejor cambiarlo a esto:

SimpleXMLElement Object (
  [item] => Array (
    [0] => SimpleXMLElement Object (
      [kategoria_6] => 1
      [opis_6] => 1
      [img] => slides/slide0.jpg
    )
    [1] => SimpleXMLElement Object (
      [kategoria_6] => 2
      [opis_6] => 2
      [img] => slides/slide1.jpg
    )
    [2] => SimpleXMLElement Object (
      [kategoria_6] => 3
      [opis_6] => 3
      [img] => slides/slide4.jpg
    )
    [3] => SimpleXMLElement Object (
      [kategoria_6] => 4
      [opis_6] => 4
      [img] => slides/slide3.jpg
    )
    [4] => SimpleXMLElement Object (
      [kategoria_6] => 7
      [opis_6] => 7
      [img] => slides/slide2.jpg
    )
  )
)

¿Cómo resolverlo?