The CURL extension for PHP >= 5.5.24 has support for HTTP/2.
You also need a libcurl installed — the underlying library that the curl functions use — with HTTP/2 support enabled. That means a libcurl newer than 7.38.0 but really, the newer the better. Libcurl has to have been built with HTTP/2 support explicitly enabled, using the --with-nghttp2 flag at compile time.