PHP download file script code example

download-example.php

<?php
echo '
<a href="http://mydomain.com/download.php?download_file=some_file.pdf">PHP download file</a>';

download-mysql-example.php

<?php
$secret = 'Add some secret strinhg here...';
$file_id = 123; // or something else you got from your MySQL database
$slug = md5($file_id.$secret);

echo '
<a href="http://mydomain.com/dowload.php?fid='.$slug.'">PHP download file via MySQL</a>';

download-mysql.php

<?php
ignore_user_abort(true);
set_time_limit(0); 
$path = "/absolute_path_to_your_files/";

$secret = 'your-secret-string';

if (isset($_GET['fid']) && preg_match('/^([a-f0-9]{32})$/', $_GET['fid'])) {
	$db = new mysqli('localhost', 'username', 'password', 'databasename');
	$result = $db->query(sprintf("SELECT filename FROM mytable WHERE MD5(CONCAT(ID, '%s')) = '%s'", $secret, $db->real_escape_string($_GET['fid'])));
	if ($result->num_rows == 1) {
		$obj = $result->fetch_object();
		$fullPath = $path.$obj->filename;
		if ($fd = fopen ($fullPath, "r")) {
			//
			// Place here the other PHP download code
			//
		}
		fclose ($fd);
		exit;
	} else {
		die('no match');
	}
} else {
	die('missing file ID');
}

download.php

<?php

ignore_user_abort(true);
set_time_limit(0); // disable the time limit for this script

$path = "/absolute_path_to_your_files/"; // change the path to fit your websites document structure

$dl_file = preg_replace("([^\w\s\d\-_~,;:\[\]\(\).]|[\.]{2,})", '', $_GET['download_file']); // simple file name validation
$dl_file = filter_var($dl_file, FILTER_SANITIZE_URL); // Remove (more) invalid characters
$fullPath = $path.$dl_file;

if ($fd = fopen ($fullPath, "r")) {
    $fsize = filesize($fullPath);
    $path_parts = pathinfo($fullPath);
    $ext = strtolower($path_parts["extension"]);
    switch ($ext) {
        case "pdf":
        header("Content-type: application/pdf");
        header("Content-Disposition: attachment; filename=\"".$path_parts["basename"]."\""); // use 'attachment' to force a file download
        break;
        // add here more headers if you use other content types
        default;
        header("Content-type: application/octet-stream");
        header("Content-Disposition: filename=\"".$path_parts["basename"]."\"");
        break;
    }
    header("Content-length: $fsize");
    header("Cache-control: private"); //use this to open files directly
    while(!feof($fd)) {
        $buffer = fread($fd, 2048);
        echo $buffer;
    }
}
fclose ($fd);
exit;

can we use it for Dropbox link?

Sure, if you use the Dropbox API. This code is only for sending a file to the browser, nothing else ;)

i created download.php and put/replace the Dropbox link at this line -> $path = "/absolute path_to_your_files/"; but returns 404 page

This is not how it works, this snippet is for files you process on your own server.
What you try is not possible with that script.