We had several support requestes about that on phpbb.de
(Just one example: https://www.phpbb.de/community/viewtopic.php?f=73&t=204889)

As per title and as per post, php >5.3 throws an E_STRICT error on the captcha code:

Deprecated: Assigning the return value of new by reference is deprecated in /***/www/includes/captcha/plugins/phpbb_recaptcha_plugin.php on line 54

	function &get_instance()

	{

		$instance =& new phpbb_recaptcha();

		return $instance;

	}

rxu finally found the part of the manual refering to that:
http://www.php.net/manual/en/language.references.whatdo.php#language.references.whatdo.assign

Since PHP 5, new returns a reference automatically, so using =& in this context is deprecated and produces an E_STRICT message.

Therefor the Deprecated error is not suppressed by phpBB