[ Posted by Aleksandar Puharic ] Example usage of interfaces in API clients classes. Doesn't communicate with actual API server but just tries to simulate the process. In reality, interface and implementing class should be in separate files with respective filenames and paths.

fiddle_026914.php

<?php

namespace InterfacesExampleApi;

/**
 * Interface ApiClientInterface
 * @package InterfacesExampleApi
 */
interface ApiClientInterface
{
    /**
     * @param string $username
     * @param string $password
     * @return mixed
     */
    public function createUser(string $username, string $password);

    /**
     * login user with created token
     *
     * @param string $username
     * @param string $password
     * @param string $token
     * @return mixed
     */
    public function loginUser(string $username, string $password, string $token);

    /**
     * Logout user via token
     *
     * @param string $token
     * @return mixed
     */
    public function logoutUser(string $token);
}

/**
 * Class ApiClient
 * @package InterfacesExampleApi
 */
class ApiClient implements ApiClientInterface
{
    /**
     * @var string
     */
    protected $apiServer;

    public function __construct(string $apiServer)
    {
        $this->apiServer = $apiServer;
    }

    /**
     * @param array $params
     */
    protected function apiCall(array $params)
    {
        return "Simulated: Sent {$params['action']} API call to {$this->apiServer}." . PHP_EOL . " Request: " . PHP_EOL . json_encode($params['params'], JSON_PRETTY_PRINT) . PHP_EOL . PHP_EOL;
    }


    public function createUser(string $username, string $password)
    {
        return $this->apiCall([
            'action' => __FUNCTION__,
            'params' => [
                'username' => $username,
                'password' => crypt($password, 'superSecretSalt123')
            ]
        ]);
    }

    public function loginUser(string $username, string $password, string $token)
    {
        return $this->apiCall([
            'action' => __FUNCTION__,
            'params' => [
                'username' => $username,
                'password' => $password,
                'token' => $token
            ]
        ]);
    }

    public function logoutUser(string $token)
    {
        return $this->apiCall([
            'action' => __FUNCTION__,
            'params' => [
                'token' => $token
            ]
        ]);
    }
}


$api = new ApiClient("https://api.example.io");

$token = bin2hex(random_bytes(32));
echo "<pre>";
echo $api->createUser("JohnSmith", "smith1234");
echo $api->loginUser("JohnSmith", "smith1234", $token);
echo $api->logoutUser($token);
echo "</pre>";
?>