<form method="post">
<input type="submit" name="submit"/>
</form>
<?php
function sendHeaders($file, $type, $name=NULL)
{
    if (empty($name))
    {
        $name = basename($file);
    }
    header('Pragma: public');
    header('Expires: 0');
    header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
    header('Cache-Control: private', false);
    header('Content-Transfer-Encoding: binary');
    header('Content-Disposition: attachment; filename="'.$name.'";');
    header('Content-Type: ' . $type);
    header('Content-Length: ' . filesize($file));
}
if(isset($_POST['submit']))
{
    $file = 'logo.png';
    if (is_file($file))
    {
        sendHeaders($file, 'image/png', 'My picture.png');
        ob_clean();
        flush();
        @readfile($file);
        exit;
    }
}
?>