Rien ne s'affiche après le chargement de la page

au lieu de

<body onload=showUser(str="ALL")>

vous devriez utiliser

<body>
<!-- all your html goes here -->
</body>
<script>
    showUser();
</script>

Lorsque body onload déclenche drop_1 et tire_two ne sont pas créés et le scrup lèvera juste une exception.

Modifier :

Lors du premier chargement ou si votre q=ALL , vous n'avez pas besoin d'envoyer le second paramètre p . alors change

if (str == "" || str1 == "") {
    document.getElementById("txtHint").innerHTML = "";
    return;
}

if (str == "" || (str != "ALL" && str1 == "")) {
    document.getElementById("txtHint").innerHTML = "";
    return;
}

Modification 2 : Combobox.php modifié

<script> // AJAX Implementation
function showUser() {
    str = document.getElementById("drop_1").value;
    str1 = document.getElementById("tier_two").value;
    if (str == "" || (str != "ALL" && str1 == "")) {
        document.getElementById("txtHint").innerHTML = "";
        return;
    }
    if (window.XMLHttpRequest) { // code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp = new XMLHttpRequest();
    } else { // code for IE6, IE5
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.onreadystatechange = function () {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
            document.getElementById("txtHint").innerHTML = xmlhttp.responseText;
        }
    }
    xmlhttp.open("GET", "getuser.php?q=" + str + "&p=" + str1, true);
    xmlhttp.send();
}
</script>
<body>

<?php include('func.php'); ?>
<select name="drop_1" id="drop_1" onchange="showUser()" style="overflow:scroll;width:100px;">
        <option value="ALL" selected='ALL'>ALL</option>
        <?php getTierOne(); ?>
</select>
<!-- your other html... -->
</body>

<script>
    showUser();
</script>

Modification 3 :

C'est une supposition folle et c'est peut-être ce que vous voulez

changements dans func.php

if(isset($_GET['func']) && $_GET['func'] == "drop_1") {
   drop_1($_GET['drop_var']); 
}else{
   drop_1(''); // when you call the func.php first time $_GET['drop_var'] is not set so call with blank drop_var
}

function drop_1($drop_var)
{
    $mysqli = new mysqli("localhost", "root", "", "app");
    $query = "SELECT * FROM app WHERE app_cn='$drop_var' GROUP BY app_plan_no ORDER BY app_plan_no";
    if (empty($drop_var)){
        //if the drop_var is empty select all records
        $query = "SELECT * FROM app GROUP BY app_plan_no ORDER BY app_plan_no";
    }
    $results = $mysqli->query($query);

    echo '<select name="tier_two" id="tier_two" onchange="showUser()">
          <option value=" " disabled="disabled" selected="selected">Choose one</option>';

          while($drop_2 = $results->fetch_assoc())
            {
            if($drop_2['app_plan_no'] != '')
            {
              echo '<option value="'.$drop_2['app_plan_no'].'">'.$drop_2['app_plan_no'].'</option>';
            }
            }
    echo '</select> ';
}