Ubuntu users:

1) Install the curl extension:

sudo apt-get install php5-curl

2) Restart the apache:

sudo apache2ctl restart

Redhat/CentOS/Fedora users:

1)Install the extension

yum install curl curl-devel

2)Restart apache

sudo service httpd restart

Windows XAMPP users

1) Open ..XAMPP\php\php.ini file

2) Find the line ;extension=php_curl.dll

3) Remove the ; character before the word extension

4) Restart the server