SimpleXML PHP - Change Value from any Node

Normaly i find everything at stackoverflow what iam locking for. but now i need ur help.

my example xml:

<xml>
  <first>
    <change>Text to change</change>
  </first>
  <second>
    <change1>Text to change</change1>
    <change2>Text to change</change2>
    <change3>Text to change</change3>
  </second>
</xml>

Now i need to change the text from the change nodes. But this is a example xml. i dont know the structure from the xml. i only have the change names. is there sth like in js getElementsByTagName("change") what is to when i want to change the text from the change nodes

thanks guys... and sry for my english ;)