$xmlstr="../XML/questions.xml";

$local = 0;
if($local == 0){
    $server ="localhost";
    $user = "root";
    $pass = "";
    $db = "quiz";
}else{
    $server ="localhost";
    $user = "id3001514_t17";
    $pass = "12345";
    $db = "id3001514_quiz";
}
$konektatu = mysqli_connect($server, $user, $pass, $db);

$sql = "INSERT INTO questions VALUES('','$_POST[posta]', '$_POST[galdera]', '$_POST[zuzena]', '$_POST[okerra1]','$_POST[okerra2]','$_POST[okerra3]', '$_POST[zailtasuna]', '$_POST[arloa]')";
if (file_exists($xmlstr)) {
    $questions =  simplexml_load_file($xmlstr);
    $simpleXML = new SimpleXMLElement($questions->asXML());
    $question = $simpleXML->addChild('assessmentItem');
    $question->addChild('itemBody','Hau galdera izango zen');
    $correct=$question->addChild('correctResponse');
    $correct->addChild('value','Hemen erantzun zuzena');
    $incorrect=$question->addChild('incorrectResponse');
    $incorrect->addChild('value','Hemen erantzun okerra');
    $incorrect->addChild('value','Hemen Bigarrena');
    $simpleXML->asXML($xmlstr);

} else {
    echo "Error, no se pudo abrir el archivo XML, revise la ruta y si existe";
}
$execute=mysqli_query($konektatu,$sql);
if(!$execute){
    echo"Ezin izan dira datuak txertatu. ".mysqli_error($konektatu);
}else{
    echo"Datuak txertatu egin dira";
    echo"<a href=../HTML5/showQuestions.php> Ikusi Datuak </a><br>";
    echo'<a href=../PHP/showXMLQuestions.php>Ikusi Datuak (XML) </a>';
}
mysqli_close($konektatu);

<?xml version="1.0"   encoding="UTF-8"  ?>
<assessmentItems>

    <assessmentItem complexity="3" subject="mikologia">

        <itemBody>

            <p>Zein Amanita da jangarria?</p>

        </itemBody>

        <correctResponse>

            <value>Caesarea</value>

        </correctResponse>

        <incorrectResponses>

            <value>Phalloides</value>

            <value>Muscaria</value>

            <value>Virosa</value>

        </incorrectResponses>

    </assessmentItem>

    <assessmentItem complexity="3" subject="mikologia">

        <itemBody>

            <p>Tripakiek zer dute kapela azpian?</p>

        </itemBody>

        <correctResponse>

            <value>Eztenak</value>

        </correctResponse>

        <incorrectResponses>

            <value>Filamenduak</value>

            <value>Himenioa</value>

            <value>Hodiak</value>

        </incorrectResponses>

    </assessmentItem>

    <assessmentItem complexity="5" subject="mikologia">

        <itemBody>

            <p>Eranztuna du</p>

        </itemBody>

        <correctResponse>

            <value>Galanpernak</value>

        </correctResponse>

        <incorrectResponses>

            <value>Gibel urdinak</value>

            <value>Esnegorriak</value>

            <value>Errotariak</value>

        </incorrectResponses>

    </assessmentItem>

</assessmentItems>

<assessmentItems>
...
    <assessmentItem>

        <itemBody>
            <p>Hau galdera izango zen</p>
        </itemBody>

        <correctResponse>
            <value>Hemen erantzun zuzena</value>
        </correctResponse>

        <incorrectResponses>
            <value>Hemen erantzun okerra</value>
            <value>Hemen Bigarrena</value>
        </incorrectResponses>

    </assessmentItem>
...
</assessmentItems>

$xmlstr='../XML/questions.xml';

if (file_exists($xmlstr)) {

$questions =  simplexml_load_file($xmlstr);
$simpleXML = new SimpleXMLElement($questions->asXML());
$question = $simpleXML->addChild('assessmentItem');
$question->addChild('itemBody','Hau galdera izango zen');
$correct=$question->addChild('correctResponse');
    $correct->addChild('value','Hemen erantzun zuzena');
$incorrect=$question->addChild('incorrectResponse');
    $incorrect->addChild('value','Hemen erantzun okerra');
    $incorrect->addChild('value','Hemen Bigarrena');

$simpleXML->asXML($xmlstr);

} else {

echo "Error, no se pudo abrir el archivo XML, revise la ruta y si existe";

}

$xmlstr='myxml.xml';

    if (file_exists($xmlstr)) {

    $questions =  simplexml_load_file($xmlstr);
    $question = $questions->addChild('assessmentItem');
    $question->addChild('itemBody','Hau galdera izango zen');
    $correct=$question->addChild('correctResponse');
        $correct->addChild('value','Hemen erantzun zuzena');
    $incorrect=$question->addChild('incorrectResponse');
        $incorrect->addChild('value','Hemen erantzun okerra');
        $incorrect->addChild('value','Hemen Bigarrena');
    $questions->asXML($xmlstr);

} else {

    echo "Error, no se pudo abrir el archivo XML, revise la ruta y si existe";

}

<assessmentItem>

    <itemBody>Hau galdera izango zen</itemBody>

    <correctResponse>
        <value>Hemen erantzun zuzena</value>
    </correctResponse>

    <incorrectResponse>
        <value>Hemen erantzun okerra</value>
        <value>Hemen Bigarrena</value>
    </incorrectResponse>

</assessmentItem>