Munges all browsable (usually http, https and ftp) absolute URIs into another URI, usually a URI redirection service. This directive accepts a URI, formatted with a %s where the url-encoded original URI should be inserted (sample: http://www.google.com/url?q=%s).
%s
http://www.google.com/url?q=%s
Uses for this directive:
Prior to HTML Purifier 3.1.1, this directive also enabled the munging of browsable external resources, which could break things if your redirection script was a splash page or used meta tags. To revert to previous behavior, please use %URI.MungeResources.
meta
You may want to also use %URI.MungeSecretKey along with this directive in order to enforce what URIs your redirector script allows. Open redirector scripts can be a security risk and negatively affect the reputation of your domain name.
Starting with HTML Purifier 3.1.1, there is also these substitutions:
<a href="">
Admittedly, these letters are somewhat arbitrary; the only stipulation was that they couldn't be a through f. r is for resource (I would have preferred e, but you take what you can get), n is for name, m was picked because it came after n (and I couldn't use a), p is for property.