Source code for download file with PHP

$FileName = "file";                                                                     
$File = "pathway/".$FileName;                                                                              

if(is_file($File)){                                                                                           
    if(strstr($HTTP_USER_AGENT, "MSIE")){                                                                
        header('Content-type: application/octet-stream');                                             
        header('Content-Disposition: attachment; filename="'.$FileName.'"');                          
        header('Expires: 0');
        header('Cache-Control: must-revalidate, post-check=0, pre-check=0');                          
        header("Content-Transfer-Encoding: binary");                                                  
        header('Pragma: public');
        header("Content-Length: ".filesize("$File"));                                                 
    }else{                                                                                                
        header("Content-type: file/unknown");                                                         
        header("Content-Length: ".filesize("$File"));                                                 
        header("Content-Disposition: attachment; filename='$FileName'");                              
        header("Content-Description: PHP3 Generated Data");                                           
        header("Cache-Control: cache, must-revalidate");                                              
        header("Pragma: no-cache"); 
        header("Expires: 0"); 
    }       

    $fp = fopen($File, "rb");                                                                             
    fpassthru($fp);
    fclose($fp); 
}