Why this code is not working on linux server?

I am new to the Zend Framework, and this code is used for downloading contents. This code is working on localhost but when I try to execute it on a linux server it shows error file not found.

public function downloadAnnouncementsAction() 
{
            $file= $this->_getParam('file');
            $file = str_replace("%2F","/",$this->_getParam('file'));

            // Allow direct file download (hotlinking)?
            // Empty - allow hotlinking
            // If set to nonempty value (Example: example.com) will only allow downloads when referrer contains this text
            define('ALLOWED_REFERRER', '');

            // Download folder, i.e. folder where you keep all files for download.
            // MUST end with slash (i.e. "/" )
            define('BASE_DIR','file_upload');

            // log downloads?  true/false
            define('LOG_DOWNLOADS',true);

            // log file name
            define('LOG_FILE','downloads.log');

            // Allowed extensions list in format 'extension' => 'mime type'
            // If myme type is set to empty string then script will try to detect mime type
            // itself, which would only work if you have Mimetype or Fileinfo extensions
            // installed on server.
            $allowed_ext = array (        
              // audio
              'mp3' => 'audio/mpeg',
              'wav' => 'audio/x-wav',

              // video
              'mpeg' => 'video/mpeg',
              'mpg' => 'video/mpeg',
              'mpe' => 'video/mpeg',
              'mov' => 'video/quicktime',
              'avi' => 'video/x-msvideo'
            );


            // If hotlinking not allowed then make hackers think there are some server problems
            if (ALLOWED_REFERRER !== ''
            && (!isset($_SERVER['HTTP_REFERER']) || strpos(strtoupper($_SERVER['HTTP_REFERER']),strtoupper(ALLOWED_REFERRER)) === false)
            ) {
              die("Internal server error. Please contact system administrator.");
            }

            // Make sure program execution doesn't time out
            // Set maximum script execution time in seconds (0 means no limit)
            set_time_limit(0);

            if (!isset($file) || empty($file)) {
             die("Please specify file name for download.");
            }

            // Nullbyte hack fix
            if (strpos($file, "\0") !== FALSE)
                    die('');

            // Get real file name.
            // Remove any path info to avoid hacking by adding relative path, etc.
            $fname =  basename($file);


            // Check if the file exists
            // Check in subfolders too
            function find_file ($dirname, $fname, &$file_path) {

              $dir = opendir($dirname);

              while ($file = readdir($dir)) {
                if (empty($file_path) && $file != '.' && $file != '..') {
                  if (is_dir($dirname.'/'.$file)) {
                    find_file($dirname.'/'.$file, $fname, $file_path);
                  }
                  else {
                    if (file_exists($dirname.'/'.$fname)) {
                      $file_path = $dirname.'/'.$fname;
                      return;
                    }
                  }
                }
              }

            } // find_file

            // get full file path (including subfolders)
            $file_path = '';
            find_file(BASE_DIR, $fname, $file_path);

            if (!is_file($file_path)) {
              die("File does not exist. Make sure you specified correct file name.");
            }

            // file size in bytes
            $fsize = filesize($file_path);

            // file extension
            $fext = strtolower(substr(strrchr($fname,"."),1));

            // check if allowed extension
            if (!array_key_exists($fext, $allowed_ext)) {
              die("Not allowed file type.");
            }

            // get mime type
            if ($allowed_ext[$fext] == '') {
              $mtype = '';
              // mime type is not set, get from server settings
              if (function_exists('mime_content_type')) {
                $mtype = mime_content_type($file_path);
              }
              else if (function_exists('finfo_file')) {
                $finfo = finfo_open(FILEINFO_MIME); // return mime type
                $mtype = finfo_file($finfo, $file_path);
                finfo_close($finfo);
              }
              if ($mtype == '') {
                $mtype = "application/force-download";
              }
            }
            else {
              // get mime type defined by admin
              $mtype = $allowed_ext[$fext];
            }

            // Browser will try to save file with this filename, regardless original filename.
            // You can override it if needed.

            if (!isset($_GET['fc']) || empty($_GET['fc'])) {
              $asfname = $fname;
            }
            else {
              // remove some bad chars
              $asfname = str_replace(array('"',"'",'\\','/'), '', $_GET['fc']);
              if ($asfname === '') $asfname = 'NoName';
            }

            // set headers
            header("Pragma: public");
            header("Expires: 0");
            header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
            header("Cache-Control: public");
            header("Content-Description: File Transfer");
            header("Content-Type: $mtype");
            header("Content-Disposition: attachment; filename=\"$asfname\"");
            header("Content-Transfer-Encoding: binary");
            header("Content-Length: " . $fsize);

            // download

            // @readfile($file_path);
            $file = @fopen($file_path,"rb");
            if ($file) {
              while(!feof($file)) {
                print(fread($file, 1024*8));
                flush();
                if (connection_status()!=0) {
                  @fclose($file);
                  die();
                }
              }
              @fclose($file);
            }

            // log downloads
            if (!LOG_DOWNLOADS) die();

            $f = @fopen(LOG_FILE, 'a+');
            if ($f) {
              @fputs($f, date("m.d.Y g:ia")."  ".$_SERVER['REMOTE_ADDR']."  ".$fname."\n");
              @fclose($f);
            }
}

Please help...

The two typical issues I often see between local Windows (XP) dev environment and a Linux production environment are:

File permissions. Linux typically has a more restrictive model of file permissions.
Case-sensitivity of the filenames: Linux filenames are case-sensitive. In Windows, they are not.
Different directory separator: Windows uses \, Linux uses /. The PHP constant DIRECTORY_SEPARATOR always contains the right value for the OS (thanks to @Svish).
Different include path separator: Windows uses ;, Linux uses :. The PHP constant PATH_SEPARATOR always contains the right value for the OS.