Rule for RSVP is producing an error - Cannot modify header information - headers already sent by

I'm using latest RSVP, Event and Rules modules, created rule for RSVP using custom PHP code. I got error every time the rule actived when I create Event:

Cannot modify header information - headers already sent by (output started at .../sites/all/modules/rules/rules/modules/php.rules.inc(107) : eval()'d code:19) in .../includes/common.inc on line 345.