problem with including external library Below I check for file existence and its exists.

$path = libraries_get_path('nusoap') . '/nusoap.php';
if(file_exists($path)) {
    print file_get_contents($path);
    die;
}


But when I try to include it I get 500 Internal server error.

$path = libraries_get_path('nusoap') . '/nusoap.php';
if(file_exists($path)) {
    require_once $path;
}


Owner and Group of all files are www-data

EDIT

[Mon Dec 24 12:59:12 2012] [error] [client 2.146.17.144] PHP Fatal error:  Cannot redeclare class XMLSchema in /sites/all/modules/fedora_store/nusoap/nusoap.php on line 2049, referer: http://127.0.0.1/fedora/store/transaction/2/mellat
When you get fatal web server/PHP error it is very useful to check the logs. Usually Apache logs are placed in /var/log/apache2/error.log but it may differ.  


  Owner and Group of all files are www-data


I must warn you that www-data should not be an owner for most PHP files. www-data usually needs only read permissions. But it is not directly related to this question.