Pecl HTTP doesn't seem to include this function, tested on latest ubuntu
1 execute
sudo pecl install raphf propro pecl_http
2 create /etc/php5/mods-available/zhttp.ini
extension=raphf.so
extension=propro.so
extension=http.so
3 execute
sudo php5enmod zhttp
4 restart your web server
Running php -r 'http_post_data();' still gives me Call to undefined function http_post_data()