CURL is a PHP extension that allows you to fetch contents of remote files, do post requests, get requests and a lot more.

Setting up CURL extension in php on IIS

Installing CURL in three easy steps on IIS
For an ISAPI Setup
Step 1:
Copy libeay32.dll from your PHP directory to the c:\windows\system32 directory
Copy ssleay32.dll from your PHP directory to the c:\windows\system32 directory
Make sure to set the permissions on both files so that IUSER_MACHINENAME can acess them

Step 2:
Open your php.ini
Find
;extension=php_curl.dll
Change to
extension=php_curl.dll

If that line does not exist just add after other extensions.

Step 3:
When running IIS in ISAPI you will need to do a quick restart of the web server service in order for any changes to the php.ini to take affect.