Using passthru() to download file results in broken/modified file

Question

I have an PHP Yii2 webapplication, which runs an command line script using passthru(). The passthru() result should passed into an downloaded file, so that the user can download the output of passthru().

Example:

If I execute the following command line:

$ /bin/cat /PATH/pic.jpg > output.jpg

...I get an file output.jpg, which is identical to pic.jpg (same md5 checksum)

But I need to execute this mechanism in an web application:

If I doing the same using passthru() in order to send the file to the users browser:

            header("Cache-Control: private");
            header("Content-Type: application/octet-stream");
            header("Content-Disposition: attachment; filename=download.jpg");
            passthru ('/bin/cat /PATH/pic.jpg');

....than I get an download.jpg which is not the same like pic.jpg I this example-case download.jpg is 1126 bytes smaller (picture size is around 13MBytes) and the picture can not be opend in some picture viewer.