POP and IMAP functionality are packaged together when it comes to PHP.
On Linux you need to install the php-imap package for your distribution.
Or compile PHP with php-imap enabled.
On Windows you need to enable the php_imap.dll that should be in PHP's ext directory. You can do this by editing the php.ini and uncommenting or adding the following:
extension=php_imap.dll
If you do not have that dll then you will have to get it. I am pretty sure that it comes standard with all recent versions of php from:
http://windows.php.net/download(http://windows.php.net/download)
If you use some other specific WAMP (bundle) package you would have to see what they say as I have no experience with using those types of packages to install, and the versions of PHP could vary. The basics are the same, but you need to make sure the version of php-imap.dll matches your php distribution.