You need to add the PHP directory to your path. On the command line (e.g. in a batch file), it would look like this:
SET PATH=%PATH%;C:\your\wamp\path\php
if in doubt, it’s the directory containing the php.exe.
You can also pre-set the path in Windows’ control panel. See here on how to do this in Windows 7 for example.
Be aware that if you call the PHP executable from an arbitrary directory, that directory will be the working directory. You may need to adjust your scripts so they use the proper directories for their file operations (if there are any).