PHP: Force download is downloading an empty file - php

Question

The code below displays this:
/home/my_site/www/wp-content/uploads/2017/03/2017-03-17-my_file.mp3
As far as I can tell, this path is correct. Yet when I comment away the echo and download the file, it's an empty file. Why?
Code:
if (isset($_GET['file'])) {
clearstatcache();
$file_path = str_replace('http://www.example.com/', '', $_GET['file']);
$file_path = $_SERVER['DOCUMENT_ROOT'] . '/' . $file_path . '.mp3';
echo $file_path;
if(file_exists($file_path)) {
$file_name = basename($file_path);
$file_size = filesize($file_path);
header("Cache-Control: private");
header("Content-Type: application/stream");
header("Content-Length: ".$file_size);
header("Content-Disposition: attachment; filename=".$file_name);
exit();
}
else {
die('The provided file path is not valid.');
}
}
EDIT: this is what I have after KIKO Software's suggestion. Still downloads empty file.
<?php
if (isset($_GET['file'])) {
$file_path = $_SERVER['DOCUMENT_ROOT'] . '/' . $_GET['file'] . '.mp3';
//echo $file_path;
//$file_path = $_SERVER['SCRIPT_FILENAME'];
if (file_exists($file_path)) {
$file_name = basename($file_path);
$file_size = filesize($file_path);
header('Content-type: application/octet-stream');
header('Content-Transfer-Encoding: binary');
header('Content-Length: '.$file_size);
header('Content-Disposition: attachment; filename='.$file_name);
//readfile($file_path);
exit();
}
else {
die('The provided file path is not valid.');
}
}
?>

Answer 1

Simple example to download a file:
$content = 'This is the content.';
$size = strlen($content);
$name = 'test.txt';
header('Content-type: application/octet-stream');
header('Content-Transfer-Encoding: binary');
header('Content-Length: '.$size);
header('Content-Disposition: attachment; filename='.$name);
echo $content;
Make sure this works first. If it doesn't there could be a problem with the headers. Switch on error reporting and see: How to fix "Headers already sent" error in PHP
Then build up on that. Start by downloading the script itself:
$file_path = $_SERVER['SCRIPT_FILENAME'];
if (file_exists($file_path)) {
$file_name = basename($file_path);
$file_size = filesize($file_path);
header('Content-type: application/octet-stream');
header('Content-Transfer-Encoding: binary');
header('Content-Length: '.$file_size);
header('Content-Disposition: attachment; filename='.$file_name);
readfile($file_path);
exit();
}
else {
die('The provided file path is not valid.');
}
And only after that try to download something else.
By approaching the problem step by step it is easier to see where it goes wrong.