lib/Message/ResponseBuilder.php
public function addHeader(string $input): void
    {
        list($key, $value) = explode(':', $input, 2);
        $this->response = $this->response->withAddedHeader(trim($key), trim($value));
    }
public function addHeader(string $input): void
    {
        list($key, $value) = explode(':', $input, 2);
        $this->response = $this->response->withAddedHeader(trim($key), trim($value));
    }
Add a single HTTP header line.