PHP

<?php
function doing_ajax(){
    return isset($_SERVER['HTTP_X_REQUESTED_WITH'])
        && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) === 'xmlhttprequest';
}

if ( doing_ajax() ){
    // I shall do some logic over here, if the logic is true,
    // then I'll tell the client that the request is success.
    echo '{status:true}';
    exit();
}

header( 'location: admin.php?loading=dismiss?callback=admin.php?upload' );
?>