For Ubuntu
Install Curl extension for PHP & restart apache server.
sudo apt-get install php5-curl
sudo service apache2 restart
For Windows
Problem arises because of not including the lib_curl.dll to PHP.
also load following extension if not working,so those extension in php.ini
or remove comment if already exist in php.ini
file then remove comment.
extension=php_bz2.dll
extension=php_curl.dll
extension=php_dba.dll
Now restart apache server. If you get an error "Unable to Load php_curl.dll", copy SSLEAY32.PHP, libEAY32.dll (OpenSSL) Libraries to the System32 folder.