Добавил ifset которая реагирует только на установленность.

klein.php

@@ -20,6 +20,7 @@ class Klein {
             ["{% for+:id+in+:var %}", "foreach(#2s as \$index => #1s): \$loop = Klein::loop(\$index, #2s);"],
             ["{% for+:id , :id+in+:var %}", "foreach(#3s as #1s => #2s):"],
             ["{% endfor %}", "endforeach;"],
+            ["{% ifset+:var %}", "if(isset(#1s)):"],
             ["{% if+:var %}", "if(isset(#1s) && #1s):"],
             ["{% unless+:var %}", "if(!(isset(#1s) && #1s)):"],
             ["{% else %}", "else:"],