How to add a child in xml without overwrite using php dom?

Solution:

as per the comment: You should check that the root node of the XML file exists before callingcreateElement to generate a new one. To do that you can callgetElementsByClassName and test whether the first entry is empty

ie:empty( $dom->getElementsByTagName('Territorio')[0] ) sort of thing...

If the root node exists we use that, otherwise add a new root to the document and continue

// check that the querystring has all the required parameters
if( isset(
    $_GET['FOL'],
    $_GET['NUM'],
    $_GET['DAT'],
    $_GET['ZON'],
    $_GET['TIP'],
    $_GET['COM']
)){
    // the filename is generated from one of the querystring parameters
    // - here the directory used is the same as the script running
    $file=sprintf( '%s/%s.xml', __DIR__, $_GET['NUM'] );
    
    // create an empty file if it does not exist
    if( !file_exists( $file )){
        file_put_contents( $file, '' );
    }
    
    clearstatcache();
    
    // create the DOMDocument and set various options
    libxml_use_internal_errors( true );
    $dom=new DOMDocument('1.0','utf-8');
    $dom->strictErrorChecking=false;
    $dom->preserveWhiteSpace=true;
    $dom->formatOutput=true;
    $dom->recover=true;
    
    // load the XML file directly rather than loading a string read from the original file.
    $dom->load( $file );
    
    // The ROOT node of the document ... does it exist? if not, create it and add to the DOM.
    $root=$dom->getElementsByTagName('Territorio')[0];
    if( empty( $root ) ){
        $root=$dom->createElement('Territorio');
        $dom->appendChild( $root );
    }
    
    // I added this part so that you can distinguish easily new elements
    $record=$dom->createElement('record');
    $root->appendChild( $record );
    
    // add all the querystring parameters within the new record.
    $record->appendChild( $dom->createElement('NUM', $_GET["NUM"] ) );
    $record->appendChild( $dom->createElement('DAT', $_GET["DAT"] ) );
    $record->appendChild( $dom->createElement('ZON', $_GET["ZON"] ) );
    $record->appendChild( $dom->createElement('TIP', $_GET["TIP"] ) );
    $record->appendChild( $dom->createElement('COM', $_GET["COM"] ) );
    $record->appendChild( $dom->createElement('FOL', $_GET["FOL"] ) );
        
        
    $dom->save( $file );
    
}
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An example of the XML generated:

<?xml version="1.0" encoding="utf-8"?>
<Territorio>
  <record>
    <NUM>wibble</NUM>
    <DAT>25/10/2022</DAT>
    <ZON>europe</ZON>
    <TIP>total</TIP>
    <COM>94</COM>
    <FOL>234</FOL>
  </record>
  <record>
    <NUM>wibble</NUM>
    <DAT>26/10/2022</DAT>
    <ZON>europe</ZON>
    <TIP>total</TIP>
    <COM>96</COM>
    <FOL>238</FOL>
  </record>
</Territorio>
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In the original code the file is saved to a location defined by another parameter in the querystring ( only just noticed that afterwards ) so rather than

$file=sprintf( '%s/%s.xml', __DIR__, $_GET['NUM'] );
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you would likely want to do:

$file=sprintf( '%s/%s.xml', $_GET['FOL'], $_GET['NUM'] );
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Solution:

The root node of an XML document is called document element and here is an property for it. So you can just check if it is undefined. However an document can have only a single document element, so will need to modify the structure of your XML - for example add a "Territori" document element.

Do not use the second argument of thecreateElement() method or the$nodeValue property. Their escaping is broken - try adding a value with an&. Use$textContent or add a text node.

In modern DOM you can even justappend() a string.

$NUM = "NUM";
$DAT = "DAT";
$ZON = "ZON";

$document = new DOMDocument('1.0', 'UTF-8');
// let the parser ignore existing indents
$document->preserveWhiteSpace = false;
$document->loadXML(getXMLString());

// fetch or create document element
$territori = $document->documentElement 
    ?? $document->appendChild($document->createElement('Territori'));

// create/append an item element 
$territori
    ->appendChild(
        $territorio = $document->createElement('Territorio') 
    );

// create/append an element and set its text content
$territorio
    ->appendChild($document->createElement('NUM'))
    ->textContent = $NUM;
// create/append an element with a text child node
$territorio
    ->appendChild($document->createElement('DAT'))
    ->appendChild($document->createTextNode($DAT));
// create/append an element and a string (DOM Level 3)
$territorio
    ->appendChild($document->createElement('ZON'))
    ->append((string)$ZON);

// enable output formatting
$document->formatOutput = true;
echo $document->saveXML();

function getXMLString() {
  return <<<'XML'
<?xml version="1.0"?>
<Territori>
  <Territorio>
    <NUM>NUM</NUM>
    <DAT>DAT</DAT>
    <ZON>DAT</ZON>
  </Territorio>
</Territori>
XML;
}
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For a more flexible approach to fetch nodes use Xpath expressions. Here is an example that checks if anTerritorio with a specificNUM value exists:

$document = new DOMDocument('1.0', 'UTF-8');
$document->loadXML(getXMLString());

$xpath = new DOMXpath($document);
if ($xpath->evaluate('count(//Territorio[NUM="NUM"]) > 0')) {
  echo "Node exists";
}
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