Use some type hinting in the comments to implement a more generic connection method for the DB handler.

app/src/Infrastructure/Api/Database/DatabaseHandlerInterface.php

@@ -6,7 +6,10 @@ use PDO;
 
 interface DatabaseHandlerInterface 
 {
-    public function getConnection(): PDO;
+    /**
+     * @return mixed The return must be an active connection to a database.
+     */
+    public function getConnection();
 
     /**
      * Get the connection string required for establishing connections to the

app/src/Infrastructure/Database/DatabaseHandler.php

@@ -19,7 +19,10 @@ class DatabaseHandler implements DatabaseHandlerInterface
         $this->pdo = new PDO($this->getDsn(), $username, $password);
     }
 
-    public function getConnection(): PDO
+    /**
+     * @return PDO For this particular Handler we will return the PDO objectg
+     */
+    public function getConnection()
     {
         return $this->pdo;
     }

app/tests/Domain/Repository/UserRepositoryTest.php

@@ -6,6 +6,8 @@ use Slovocast\Domain\Repository\User\UserRepository;
 use Slovocast\Infrastructure\Api\Database\DatabaseHandlerInterface;
 use Slovocast\Infrastructure\User\BasicUserAuthorization;
 use Slovocast\Tests\TestCase;
+use Prophecy\Argument;
+
 
 class UserRepositoryTest extends TestCase
 {
@@ -14,7 +16,10 @@ class UserRepositoryTest extends TestCase
         $user = $this->getUserFromArray();
         $databaseHandler = $this->prophesize(DatabaseHandlerInterface::class);
         
-        $databaseHandler->insert()->willReturn(true);
+        $databaseHandler->insert(
+            Argument::type('string'),
+            Argument::type('array')
+        )->willReturn(true);
         $databaseHandler->getConnection()->willReturn(new class {
             public function lastInsertId(): int
             {