How can I create a Download Page with post php method?

<?php 
    $fileId = $_GET['fileId'];
    switch($fileId){
        case 1:
            $fileName = 'myFile.pdf';
            break;
        case 2:
            $fileName = 'myOtherFile.doc';
            break;
        default:
        $fileName = null;
    }
    if($fileName){
        // $fileName is your file identifier
        $fileFullPath = $_SERVER['DOCUMENT_ROOT'].'mySecretFolder/files/'.$fileName;

        if (file_exists($fileFullPath)) {
            header('Content-Description: File Transfer');
            header('Content-Type: application/octet-stream');
            header('Content-Disposition: attachment; filename='.basename($fileFullPath));
            header('Content-Transfer-Encoding: binary');
            header('Expires: 0');
            header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
            header('Pragma: public');
            header('Content-Length: ' . filesize($fileFullPath));
            ob_clean();
            flush();
            readfile($fileFullPath);
            exit;
        }
    }
?>

<a href="https://www.my-domain.com/download.php?fileId=1"> Download PDF </a>