curl is an extension that needs to be installed, it’s got nothing to do with the PHP version.
http://www.php.net/manual/en/curl.setup.php
On current versions of Debian and Ubuntu, you can likely solve this by installing the Curl extension for PHP, and restarting the webserver. Assuming the webserver is Apache 2:
sudo apt-get install php5-curl
sudo /etc/init.d/apache2 stop
sudo /etc/init.d/apache2 start