整个RSS从iso-8859-1进入UTF-8

I'm working on an Amazon Echo Skill, which is based on an RSS feed. This feed is encoded in iso-8859-1, but needs to be in UTF-8.

Because the skill only needs the <encoded> tag in my case, i tried:

$content = $xml->getElementsByTagName("encoded")
                ->item($i)->nodeValue;
utf8_encode($content);

But this didn't do anything. Also in the header when i load the file via:

$file = 'old.xml';
    $xml = new DOMDocument('1.0', 'utf-8');
    $xml->load($file);

It still says: <?xml version="1.0" encoding="iso-8859-1"?>

Now i can't find a way for solving this problem. Maybe change the whole feed to UTF-8. Any ideas?

Answer found. I loaded the feed with:

$feed = file_get_contents(' .... ');

and encoded it with:

$feed = utf8_encode($feed); 
$feed = str_replace('encoding="iso-8859-1"', 'encoding="utf-8"', $feed);

Now works fine for me.

Also i changed the load-function to:

$xml = new DOMDocument('1.0', 'utf-8');
$xml->loadXML($feed);