pecl_http 2+ won't provide http_ functions any more. They moved to Http namespace, sadly no backwards compat.
To sum it up - As of pecl_http >=2.0.0 this is no longed available. Also pecl_http 1.7.6 will work only for PHP <=5.5. For PHP 5.6 we are forced to use version pecl_http 2.0.6+