If you're on Windows:
Go to your php.ini file and remove the ; mark from the beginning of the following line:
;extension=php_curl.dll
After you have saved the file you must restart your HTTP server software (e.g. Apache) before this can take effect.
For Ubuntu 13.0 and above, simply use the debundled package. In a terminal type the following to install it and do not forgot to restart server.
sudo apt-get install php-curl
Or if you're using the old PHP5
sudo apt-get install php5-curl
or
sudo apt-get install php5.6-curl
Then restart apache to activate the package with
sudo service apache2 restart