I am gonna ressurect this dead thread because I was looking for answers about using conditional operators on placeholders/template variables:
If you are a running a newer version of Revo, PHx is built into the core and you cannot download the extra (well done to the MODx guys for including it)
Here is the documentation that helped me and taught me how to use the conditionals on placeholders ->
https://docs.modx.com/revolution/2.x/making-sites-with-modx/customizing-content/input-and-output-filters-(output-modifiers)
This thread jumps up in Google search so I thought people should know.
EDIT - Arghh, the forums doesn't allow me to link the documenration because it has "()" chars - here is the google page (top result) ->
https://www.google.com/search?q=modx+filter+output&rlz=1C1EKKP_enUS741US741&oq=modx+filter+output&aqs=chrome..69i57j0l5.3679j0j4&sourceid=chrome&ie=UTF-8
EDIT 2 - guys, you should seriously fix the forum. The above link doesn't list either.
OK so Google Input and Output Filters (Output Modifiers) | MODX Revolution and click the documentation.... Bah...