fetching results through ajax in wordpress using wpdb class

I am working with wordpress. In wordpress I make a page ‘dm_page.php’ and in that use contact form 7 plugin in which I created the dropdownlist. On change event of dropdownlist I am hitting ajax. The code for ajax I wrote in header.php. The file to which ajax is giving the data is ‘getdata.php’ placed in theme folder. I am giving the code of ‘getdata.php’.

<?php
global $wpdb;

$ddlval = $_POST['ddlval'];
$results = $wpdb->get_row('select * from my_dynamictest where lang="'.$ddlval.'"', ARRAY_A);

$someArray = [];
array_push($someArray,[
    'id' => $results['id'],
    'name' => $results['name']
]);

echo json_encode($someArray);

?>

I am getting following error from this file in my ajax error callback:
Fatal error: Call to a member function get_row() on null in…
I am not able to figure this out what the error says. Please help. Consider me as total beginner in wordpress