php - PHP In ajax responseText take all the html cose instead of ONLY the echo passed trought PHP

Question

Using that javascript ajax function I pass the content of a form, that contain the dato value, to the PHP login.php than trought the echo pass back the content (the insert form) that I want to be switched to the cancel form, using the content respondText (that may take only the echo of the PHP).

BUT INSTEAD the responseText contain ALL the html code, with the old html plus the cancella_form passed by the echo, that's also out of the div with id=visibile.

Any ideas why? D:

//ajaxSubmit(dato)
function ajaxSubmit( url , divId , hideId ) {

            //in setXmlHttpObject() I just control the user's browser
            // and assign the right XmlHttp Object
    var ajaxRequest = setXmlHttpObject();

    var dato = 'nome='+document.getElementsByName('dato')[0].value;
    ajaxRequest.open("POST", url, true);
    ajaxRequest.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    ajaxRequest.send(dato);

    ajaxRequest.onreadystatechange = function() {
        //Comunication complete
        if (ajaxRequest.readyState == 4 && ajaxRequest.status==200) {
            //Comuncation succesfull
            if(ajaxRequest.statusText === "OK"){
                var str= ajaxRequest.responseText;//<<<HERE///////
                $(str).replaceAll("#visibile");

            }
            //Comuncation failed
            else{
                var str= "ERROR: Ajax: "+ajaxRequest.responseText;
                document.write(str);


            }
        }
    }

}//FINE ajaxRequest();



<?php

    include("prova_login_adv.php");

    $conn= mysql_connect('localhost','root','');
    mysql_select_db('db_prova',$conn ) or die(mysql_error());

    //
    if(isset($_POST['nome'])){
        $dato= $_POST['nome'];
        mysql_query(" INSERT INTO test (valore) VALUES ('$dato') ") or die(mysql_error());

            /// NOW I declare what I want to be replaced in the div id="visibile"
        echo "

            <form id='form_cancella' name='form_cancella' action='' methos='POST' onSubmit=' return false;' >
              <text name='dato' value='".$dato."' >Benvenuto <b>".$dato."</b></text>
              <input type='submit' name='cancella' value='cancella' onClick=\" ajaxSubmit('logout.php','visibile','form_cancella');\" />
            </form>

        ";

    }

?>