Add line to xml file through php in AS3

im creating a very simple CMS panel for my website(i'm still lerning). And I have some problems. I've created the panel in Flash-AS3, which loads the php file, which opens and saves the xml data. Here are the codes:

AS3 code:

function onClicked(e:MouseEvent):void
{
    var myXmlString:String = "<?xml version=\"1.0\" encoding=\"utf-8\"?><NEWSCONTENT><NEWS ID=\"" + idTxt.text + "\" IMG=\"" + sourceTxt.text +"\" TITLE=\"" + titleTxt.text + "\" DATE=\"" +   dateTxt.text + "\" CONTENT=\"" + contentTxt.text + "\"/></NEWSCONTENT>";

    trace(myXmlString);

    var myXml:XML = new XML(myXmlString);

    var req:URLRequest = new URLRequest("phpXML.php");
    req.data = myXml;
    req.contentType = "text/xml";
    req.method = URLRequestMethod.POST;

    var loader:URLLoader = new URLLoader();

    function onPHPLoaded(e:Event):void
    {
        statusTxt.text = "Dane zostaly zaktualizowane!";
    }
loader.load(req);
loader.addEventListener(Event.COMPLETE, onPHPLoaded);
}

send_btn.addEventListener(MouseEvent.CLICK, onClicked);

PHP code:

<?php

if (isset($GLOBALS["HTTP_RAW_POST_DATA"])) {
    $xml = $GLOBALS["HTTP_RAW_POST_DATA"];
    $file = fopen("www.xml", "w");
fwrite($file, $xml);
    fclose($file);
    echo ($GLOBALS["HTTP_RAW_POST_DATA"]);
}

?>

XML code:

<?xml version="1.0" encoding="utf-8"?>
<NEWSCONTENT>

    <NEWS ID="1" IMG="galerry/1b.jpg" TITLE="Let's begin!" DATE="23-11-2014" CONTENT="This is the content"/>

</NEWSCONTENT>

Everything works fine without errors, but it's not working the way I want to.

Now when I put data/"strings" to the text fields it removes all xml data and saves it again with the text i wrote. But i want to to add a next line "<NEWS ID="1" IMG="galerry/1b.jpg" TITLE="Let's begin!" DATE="23-11-2014" CONTENT="This is the content"/>" to the xml file.

I tried to change the: $file = fopen("www.xml", "w"); to $file = fopen("www.xml", "a");, but this added whole xml code again to the first.

Can anyone help?