src/Http/Message.php
public function getHeader($name)
    {
        $headers = [];
        $name = strtolower((string)$name);

        foreach ($this->headers as $k => $v) {
            if ($name == strtolower($k)) {
                $headers = array_merge($headers, $v);
            }
        }

        return array_unique($headers);
    }
public function getHeader($name)
    {
        $headers = [];
        $name = strtolower((string)$name);

        foreach ($this->headers as $k => $v) {
            if ($name == strtolower($k)) {
                $headers = array_merge($headers, $v);
            }
        }

        return array_unique($headers);
    }
Retrieves a message header value by the given case-insensitive name.
This method returns an array of all the header values of the given
case-insensitive header name.
If the header does not appear in the message, this method MUST return an
empty array.

@param string $name Case-insensitive header field name.
@return string[] An array of string values as provided for the given
header. If the header does not appear in the message, this method MUST
return an empty array.