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Segui todos os passos no vídeo, mas ainda assim aparece isso:

.\src\Buscador.php:8 PhanRedefineClass Class \Alura\BuscadorDeCursos\Buscador defined at .\src\Buscador.php:8 was previously defined as Class \Alura\BuscadorDeCursos\Buscador at src\Buscador.php:8
.\src\Buscador.php:10 PhanUndeclaredTypeProperty Property \Alura\BuscadorDeCursos\Buscador->httpClient has undeclared type \Psr\Http\Client\ClientInterface (Did you mean to configure a stub with https://github.com/phan/phan/wiki/How-To-Use-Stubs#internal-stubs or to enable the extension providing the class. or interface \GuzzleHttp\ClientInterface)
.\src\Buscador.php:14 PhanUndeclaredTypeParameter Parameter $httpClient has undeclared type \Psr\Http\Client\ClientInterface (Did you mean to configure a stub with https://github.com/phan/phan/wiki/How-To-Use-Stubs#internal-stubs or to enable the extension providing the class. or interface \GuzzleHttp\ClientInterface)
.\src\Buscador.php:22 PhanUndeclaredClassMethod Call to method request from undeclared class \Psr\Http\Client\ClientInterface (Did you mean to configure a stub with https://github.com/phan/phan/wiki/How-To-Use-Stubs#internal-stubs or to enable the 
extension providing the class. or interface \GuzzleHttp\ClientInterface)
src\Buscador.php:8 PhanRedefineClass Class \Alura\BuscadorDeCursos\Buscador defined at src\Buscador.php:8 was previously defined as Class \Alura\BuscadorDeCursos\Buscador at .\src\Buscador.php:8
src\Buscador.php:10 PhanUndeclaredTypeProperty Property \Alura\BuscadorDeCursos\Buscador,1->httpClient has undeclared type \Psr\Http\Client\ClientInterface (Did you mean to configure a stub with https://github.com/phan/phan/wiki/How-To-Use-Stubs#internal-stubs or to enable the extension providing the class. or interface \GuzzleHttp\ClientInterface)
src\Buscador.php:14 PhanUndeclaredTypeParameter Parameter $httpClient has undeclared type \Psr\Http\Client\ClientInterface (Did you mean to configure a stub with https://github.com/phan/phan/wiki/How-To-Use-Stubs#internal-stubs or to enable the extension providing the class. or interface \GuzzleHttp\ClientInterface)

O problema era na classe Buscador. Comparei meu arquivo com o do instrutor e havia algumas diferenças:

Estava use Psr\Http\Client\ClientInterface; ao invés de use GuzzleHttp\ClientInterface;

E faltou isso também

...
/**
 * @var ClientInterface 
 */
private $httpClient;
...

Agora só aponta dois erros:

.\src\Buscador.php:8 PhanRedefineClass Class \Alura\BuscadorDeCursos\Buscador defined at .\src\Buscador.php:8 was previously defined as Class \Alura\BuscadorDeCursos\Buscador at src\Buscador.php:8
src\Buscador.php:8 PhanRedefineClass Class \Alura\BuscadorDeCursos\Buscador defined at src\Buscador.php:8 was previously defined as Class \Alura\BuscadorDeCursos\Buscador at .\src\Buscador.php:8