code: public function header($header = [])
  	{
  		if (is_array($header))
  			$this->headers = array_merge($this->headers, $header);
  		
  		return is_string($header) ? ($this->headers[$header] ?? null) : $this->headers;
  	}
docs: Get or set header
  
  If $header is a string, the method returns the associated value. in any other case, the full array of headers is returned
  if $header is an array, the list of headers is appended to the current one
  
  @param string|null|array $header name of header to retrieve, null or list of headers to append
  
  @return string|array