admin/environment/php extension/curl

To install the curl library on Windows

1. Open the php.ini file found in the moodle/apache/bin folder
2. Find the line: ;extension=php_curl.dll
3. Remove the ; at the beginning of the line
4. Restart apache if necessary

To install the curl library on Unix

1. You need to recompile PHP from source
2. add --with-curl to the command line when you run configure

To install the curl library on Ubuntu and Debian

1. aptitude install php5-curl
2. Restart apache if necessary: sudo /etc/init.d/apache2 restart (possibly sudo /etc/init.d/apache restart ?)