Error forcing download: Cannot Modify header information

Question

By clicking a link, I wanted to download a file. The file path comes from the database. The file download path is correct, something like "catalogo/catalogo_janeiro.pdf".

<?php
$sql = mysql_query("SELECT * FROM tbl_catalogo WHERE id = '1'");
$row = mysql_fetch_array($sql);
$catalogo = $row['link']; // CAMINHO DO FICHEIRO

header("Content-disposition: attachment; filename=$catalogo");
header("Content-type: application/pdf");
readfile("$catalogo");
?> 

The link to the download:

<a href="<?php echo $catalogo;?>" target="_blank"><img src="images/catalogo.jpg" /></a>

The problem is it gives me error in all header lines();

Warning: Cannot modify header information -
headers already sent by
(output started at /home/inforcyb/public_html/opaco/header.php:15)
in /home/inforcyb/public_html/opaco/index.php on line 181

Answer 1

Warning: Cannot modify header information - headers already sent by
(output started at /home/inforcyb/public_html/opaco/header.php:15) in
/home/inforcyb/public_html/opaco/index.php on line 181

HTTP headers are the first data to be transferred on a connection. If other data is transferred before then it is no longer possible to transfer any header, preventing the function header() work.

Make sure that headers are sent before any other content: the error itself shows that at line 15 of header.php data has already been sent. Change its structure so that the headers are first sent, then the contents.

Answer 2

Create the pdf_server.php file with:

<?php
header("Content-Type: application/octet-stream");

$file = $_GET["file"] .".pdf";
header("Content-Disposition: attachment; filename=" . urlencode($file));   
header("Content-Type: application/octet-stream");
header("Content-Type: application/download");
header("Content-Description: File Transfer");            
header("Content-Length: " . filesize($file));
flush(); // this doesn't really matter.
$fp = fopen($file, "r");
while (!feof($fp))
{
    echo fread($fp, 65536);
    flush(); // this is essential for large downloads
} 
fclose($fp); 

?>

On the page where you have the download link:

<?php
$sql = mysql_query("SELECT * FROM tbl_catalogo WHERE id = '1'");
$row = mysql_fetch_array($sql);
$catalogo = $row['link'];
$link = "pdf_server.php?file=".$catalogo;
?> 

<a href="<?php echo $link;?>" target="_blank"><img src="images/catalogo.jpg" /></a>