Constants are from version 4.4, we support lower versions

src/Kdyby/RabbitMq/Command/BaseConsumerCommand.php

@@ -88,7 +88,7 @@ protected function execute(InputInterface $input, OutputInterface $output): int
 	{
 		$this->consumer->consume($this->amount);
 
-		return \Symfony\Component\Console\Command\Command::SUCCESS;
+		return 0;
 	}
 
 	/**

src/Kdyby/RabbitMq/Command/PurgeConsumerCommand.php

@@ -44,7 +44,7 @@ protected function execute(InputInterface $input, OutputInterface $output): int
 		$consumer = $this->connection->getConsumer($input->getArgument('name'));
 		$consumer->purge();
 
-		return \Symfony\Component\Console\Command\Command::SUCCESS;
+		return 0;
 	}
 
 }

src/Kdyby/RabbitMq/Command/RpcServerCommand.php

@@ -46,7 +46,7 @@ protected function execute(InputInterface $input, OutputInterface $output): int
 		$rpcServer = $this->connection->getRpcServer($input->getArgument('name'));
 		$rpcServer->start($amount);
 
-		return \Symfony\Component\Console\Command\Command::SUCCESS;
+		return 0;
 	}
 
 }

src/Kdyby/RabbitMq/Command/SetupFabricCommand.php

@@ -47,7 +47,7 @@ protected function execute(InputInterface $input, OutputInterface $output): int
 			}
 		}
 
-		return \Symfony\Component\Console\Command\Command::SUCCESS;
+		return 0;
 	}
 
 }

src/Kdyby/RabbitMq/Command/StdInProducerCommand.php

@@ -43,7 +43,7 @@ protected function execute(InputInterface $input, OutputInterface $output): int
 
 		$producer->publish(\serialize($data));
 
-		return \Symfony\Component\Console\Command\Command::SUCCESS;
+		return 0;
 	}
 
 }