Either compile the extension by yourself using these instructions. Applicable for whatever PHP version you're using.
Alternatively, prebuilt binaries are available for PHP 5.3/5.4 or PHP 5.5. Always download x86 version of the extensions (unless you're using 64bit PHP) and verify that Zend Extension Build and PHP Extension Build correspond to the version you're downloading.
Newer binaries are here: http://windows.php.net/downloads/pecl/snaps/redis/2.2.5/
After you've aquired the two DLLs, put them inside ext
folder in your PHP installation and modify php.ini, adding the following:
; php-redis