Call to undefined function curl init()

You can get errors that look like this Fatal error: Call to undefined function curl_init in foo.php on line 61 if you do not have the curl package installed.

How to install php-curl:
 * If you are using PHP on someone else's computer, e.g., you have a hosting provider, you will need to ask them to install php-curl.
 * Under Ubuntu, install the package php4-curl or php5-curl with apt-get or synaptic, then restart apache.
 * Under Windows, php-curl can be enabled by uncommenting the curl extension line in the php.ini file (--extension=php_curl.dll) and restarting the web server. Note: not sure about the exact syntax, it might be only one dash.

Do not forget to restart your Web server.

You can see if you have curl installed by creating a file test.php 

On apache2 under Ubuntu, there is a table entry that looks like this:

Thanks to mab on programmingtalk.com for the reminder of phpinfo and instructions for a Windows machine.