Check your server logs and see if there is an error related to the require_once('config.php') in Google_Client.php (If the file wasn't found, the script should have stopped).
When you do your require_once('Google_Client.php'), the following code is executed from that file. After you do your require, $apiConfig should be visible to your script.
// hack around with the include paths a bit so the library 'just works'
set_include_path(dirname(__FILE__) . PATH_SEPARATOR . get_include_path());
require_once "config.php";
// If a local configuration file is found, merge it's values with the default configuration
if (file_exists(dirname(__FILE__) . '/local_config.php')) {
$defaultConfig = $apiConfig;
require_once (dirname(__FILE__) . '/local_config.php');
$apiConfig = array_merge($defaultConfig, $apiConfig);
}
Note that you do not touch config.php. If you need to override anything in there, you create local_config.php.
From my system with PHP 5.3 I used this script. The script as show below throws no errors. Unsetting the $apiConfig replicates your error.
<?php
require_once('src/Google_Client.php');
print_r($apiConfig);
// uncommenting the next line replicates issue.
//unset($apiConfig);
$api = new Google_Client();
?>