LibXMLError XML declaration allowed only at the start of the document

From the image and code in the question is seems like what you have in your document is this:

<xmp>  <?xml version="1.0"?>  <workout>

That is, the root element of the document you’re feeding to simplexml_load_string is <xmp>, not <workout>. So LibXML starts parsing that document from the <xmp> element, then hits the <?xml version="1.0"?> and says, Hey that’s an XML declaration—that shouldn’t be here.

To prevent the XML declaration from getting added, you can replace your $xml->saveXML() with:

foreach ($xml->childNodes as $node) {   echo $xml->saveXML($node);}

Or see some of the other answers for the questions PHP DomDocument output without <?xml version="1.0" encoding="UTF-8"?> and remove xml version tag when a xml is created in php.

The idea is just that instead of outputting the whole thing as a document, your code instead iterates through all nodes of the document starting with the root and outputs them one-by-one in order.

So because you’re outputting individual nodes that way instead of the whole document as a document, that skips the unwanted XML declaration at the beginning for the document.

An alternative that may also work is, make saveXML(…) only emit the document element (root):

$xml->saveXML($xml->documentElement)

If it works, it’ll emit your workout element and all its descendants, skipping the XML declaration.