<form action="main_menu.php" method="post" data-ajax="false">
        <label for="basic">Username:</label>
        <input type="text" name="username" id="usernameID" value=""  />

        <label for="basic">Password:</label>
        <input type="password" name="password" id="passwordID" value="" />

        <div class="ui-block-b">
            <button type="submit" data-theme="a">Login</button>
        </div>
    </form>

Notice:  Undefined index: username in D:xampphtdocsuls_webmain_menu.php on line 4
Notice:  Undefined index: password in D:xampphtdocsuls_webmain_menu.php on line 5
Notice:  Undefined variable: password in D:xampphtdocsuls_webmain_menu.php on line 6

require_once("db_connect.php");

$username = addslashes(htmlentities($_POST['username'], ENT_QUOTES));
$pass = addslashes(htmlentities($_POST['password'], ENT_QUOTES));
$sql = "SELECT * FROM tbl_application_form WHERE email ='".$username."' AND is_approved ='1' AND passwd='".$pass."'";