I\'m trying to do a very simple:
require_once \'vendor/google/src/Google/Client.php\';
require_once \'vendor/google/src/Google/Service/Plus.php\';
For your problem you need to add the base directory of the library in the php 'include_path'.
Try putting this line of code before the require_once
set_include_path( get_include_path() . PATH_SEPARATOR . 'vendor/google/src' );
Above didn't work for my WordPress plugin, but I managed to solve it like this:
In Client.php
before require_once
block add:
set_include_path( plugin_dir_path( __FILE__ ) . '../' );
Hope it helps.