请检索存储在blob字段中的PDF

please can any one help with a PHP/Mysql script that can enable the download of PDF stored in a blob when a link 'DOWNLOAD NOW' is clicked. i have tried but no result. kindly help Thanks

$pdf = ... // data retrieved from BLOB field
header("Content-Type: application/pdf");
header('Content-Disposition: attachment; filename="THE_NAME_OF_THE_FILE_TO_BE_DOWNLOADED.pdf"');
echo $pdf;