Downloading a file via PHP script results in wrong/different md5 checksum - why?

Question

I'm trying to implement an indirect download through PHP. On the client side I verify if the downloaded file is correct or not using md5.

When I download the file directly (http://server/folder/file.apk) I get the same md5 checksum as on the file system, but when I download it via the PHP script (http://server/some_page.php) I get a totally different checksum. Why?

Here's my PHP script:

<?php
$name_file="test2.apk";
$path="/home/user/public_html/apk/"; 
$dimension_file=(string)filesize($name_file);

header("Content-Type: application/vnd.android.package-archive ; name=".$name_file);
header("Content-Transfer-Encoding: binary");
header("Content-Length: ".$dimension_file);
header("Content-Disposition: inline; filename=".$name_file);
header("Expires: 0");
header("Cache-Control: no-cache, must-revalidate");
header("Cache-Control: private");
header("Pragma: public");

readfile($path.$name_file);
?>

Answer 1

I found the error:

 $name_file="test2.apk";
 $path="/home/user/public_html/apk/"; 
 $dimension_file=(string)filesize($name_file); //<-- HERE! --- 

i was retrieving the size using only the name of file instead of using the full path

filesize($name_file) ---> filesize( $path . $name_file) 

the error was hidden from

header("Content-Type: application/vnd.android.package-archive"); 

and the php error response added to the content of the downloaded file.

So i suggest to who has this kind of problems to comment the "Content-Type" while debugging to see if there are some errors in the php code and when all code seems to work re-enable the "Content-Type" header.

On my server spaces in the code before the

readfile($path.$name_file);

has no influence on the checksum

Thanks to Vladimir and Rocket for good practice tips

Answer 2

I was having similar problems downloading PDF files, via an fread (although readfile gave the same problem). My eventual problem was solved by turning off the debug output of the application - which in my case was Wordpress. I had WP_DEBUG set to true which caused the differences in MD5.