Call file / class from ThirdParty folder

PHP Code:

$psr4 = [
 ...
 ...
 ...
 'TP\Tools' => APPPATH . 'ThirdParty/Foo.php'
 ]; 


The part shown above is not correct. The value should be a path to a folder - NOT to a file.

While trying to reproduce your problem here is what worked for me. Both
/app/ThirdParty/Foo.php and app/Controllers/User.php remain as you show in the OP. In fact, the only change is to what @MGatner did in /app/Config/Autoload.php. Here what I tried and it works.

PHP Code:

$psr4 = [
    'Config'      => APPPATH . 'Config',
    APP_NAMESPACE => APPPATH,                // For custom namespace
    'App'         => APPPATH,                // To ensure filters, etc still found,
    'TP\Tools'    => APPPATH . 'ThirdParty',
];