How append childs to xml in php

I'm writing a sort of little CMS in PHP. Everything works well, but I've never worked with XML, usually I use SQL DB or CSV. Now I have to append some elements to an XML sheet. I have a function that overwrite completely the file. I'd like to append data.

This is the function "write"

function scrivi() {
    if ( isset($_POST["titolo"])) {
        $doc = new DOMDocument(); 
        $doc->formatOutput = true; 
        $r = $doc->createElement( "eventi" ); 
        $doc->appendChild( $r );

        $b = $doc->createElement( "evento" ); 
        $titolo = $doc->createElement( "titolo" ); 
        $titolo->appendChild( $doc->createTextNode( $_POST['titolo'] ) ); 
        $b->appendChild( $titolo ); 
        $data = $doc->createElement( "data" ); 
        $data->appendChild( $doc->createTextNode( $_POST['data'] ) ); 
        $b->appendChild( $data ); 
        $desc = $doc->createElement( "desc" ); 
        $desc->appendChild( $doc->createTextNode( $_POST['desc'] ) ); 
        $b->appendChild( $desc ); 

        $r->appendChild( $b ); 

        $doc->save("eventi.xml");
    }
}

This is an explicative XML

<?xml version="1.0"?>
<eventi>
  <evento>
    <titolo>Festa</titolo>
    <data>02/03/1993</data>
    <desc>Centro Sociale Asilo Politico presenta 99 posse</desc>
  </evento>
  <evento>
    <titolo>Festona</titolo>
    <data>15/08/1996</data>
    <desc>Falò alla baia: solite facce, solita feccia</desc>
  </evento>
</eventi>

I also have a function that, obviously, read data from the XML. I'm able to use the "read" function, from inside the "write" one, so I can read the content, one element at time (with foreach) and insert it in the queue of element to write ($b). It works but I'm not happy of this, I'd like to simply append, or at least read the whole content of the file in a while. I tried copying some snippets here and there but I doesn't find a function I completely understand, and I NEED to understand.

Hope I was specific, Sorry for bad english.