curl is an extension that needs to be installed, it’s got nothing to do with the PHP version.
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