src/Message/Message.php
public function getHeader($name)
    {
        $key = $this->headerKey($name);
        if (array_key_exists($key, $this->headers)) {
            return $this->headers[$key];
        }

        return [];
    }
public function getHeader($name)
    {
        $key = $this->headerKey($name);
        if (array_key_exists($key, $this->headers)) {
            return $this->headers[$key];
        }

        return [];
    }
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.