Fatal error: Call to undefined function curl_init()

On old versions of Debian and Ubuntu, you solved 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 service apache2 restart

On newer versions, the package name as changed:

sudo apt install php-curl

It's possible you'll need to install more:

sudo apt-get install curl libcurl3 libcurl3-dev;

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

Don't have enough reputation to comment yet. Using Ubuntu and a simple:

sudo apt-get install php5-curl
sudo /etc/init.d/apache2 restart

Did NOT work for me.

For some reason curl.so was installed in a location not picked up by default. I checked the extension_dir in my php.ini and copied over the curl.so to my extension_dir

cp /usr/lib/php5/20090626/curl.so  /usr/local/lib/php/extensions/no-debug-non-zts-20090626

Hope this helps someone, adjust your path locations accordingly.