CommandInterface suggestion

CurrentOptionA.php

abstract class InstallationCommand extends Command
{
    abstract public function install();
}

class MyInstaller extends InstallationCommand
{
    public function install()
    {
        // do stuff
    }
}

class ThirdPartyCommand extends Command {}

class ThirdPartyInstaller extends InstallationCommand
{
    // a bunch of methods copy pasted from ThirdPartyCommand
    public function install()
    {
        // do stuff
    }
}

class InstallerConsumingCommand extends Command {
    public function addInstaller(InstallationCommand $installer) {
        $this->installers[] = $installer;
    }
}

CurrentOptionB.php

interface InstallationCommandInterface extends CommandInterface
{
    public function install();
}

class MyInstaller extends Command implements InstallationCommandInterface
{
    public function install()
    {
        // do stuff
    }
}

class ThirdPartyCommand extends Command {}

class ThirdPartyInstaller extends ThirdPartyCommand implements InstallationCommandInterface
{
    public function install()
    {
        // do stuff
    }
}

class InstallerConsumingCommand extends Command {
    public function addInstaller($installer) {
        if (!($installer instanceof Command && $installer instanceof InstallationCommandInterface)) {
          throw new \InvalidArgumentException('You need to extend Command and implement InstallationCommandInterface');
        }
        $this->installers[] = $installer;
    }
}

IdealScenario.php

interface InstallationCommandInterface extends CommandInterface
{
    public function install();
}

class MyInstaller extends Command implements InstallationCommandInterface
{
    public function install()
    {
        // do stuff
    }
}

class ThirdPartyCommand extends Command {}

class ThirdPartyInstaller extends ThirdPartyCommand implements InstallationCommandInterface
{
    public function install()
    {
        // do stuff
    }
}

class InstallerConsumingCommand extends Command {
    public function addInstaller(InstallationCommandInterface $installer) {
        $this->installers[] = $installer;
    }
}