Answers for "composer [ ' -f vendor/autoload.php ]"

composer autoload

require __DIR__ . '/vendor/autoload.php';

$log = new MonologLogger('name');
$log->pushHandler(new MonologHandlerStreamHandler('app.log', MonologLogger::WARNING));
$log->addWarning('Foo');

vendor/autoload.php download

# Create composer.json by initializing composer by:
composer init
# Create vendor/autoload.php by:
composer install

# Make sure you installed composer, if you haven't use:
sudo apt install composer # Linux
# For a other OS visit: https://getcomposer.org/download/