try this:
require_once __DIR__.'/vendor/autoload.php';
your code is:
require_once __DIR__.'C:/xampp/htdocs/vendor/autoload.php';
you dont need to specify the full path of the files (‘c:/xampp/…’)
__DIR__ will give you the current directory of the file you wrote your codes
oh and anyway, did you edit the autoload.php? if you use third party classes or plugins, you should not have to edit their core files.
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solved Fatal error: Uncaught Error: using graphaware for PHP