system/HTTP/Message.php
public function getHeader(string $name)
	{
		$orig_name = $this->getHeaderName($name);

		if (! isset($this->headers[$orig_name]))
		{
			return null;
		}

		return $this->headers[$orig_name];
	}
public function getHeader(string $name)
	{
		$orig_name = $this->getHeaderName($name);

		if (! isset($this->headers[$orig_name]))
		{
			return null;
		}

		return $this->headers[$orig_name];
	}
Returns a single header object. If multiple headers with the same
name exist, then will return an array of header objects.

@param string $name

@return array|\CodeIgniter\HTTP\Header