What's included in the previous 2 answers is "true", but IMO there is yet another possibility to "deal with this situation" (as in your question). I.e. try to come up with an alternative solution that doesn't require any "custom PHP".
Have a look at this quote from "Create a Rule to Evaluate If a Logged-in User Has Created Content of Type X" (= the HowTo you linked to in your question):
... the SQL query checks the database for a user who has created one or more nodes of the type configuration_1. If one or more nodes exist for this user, the condition evaluates to false ...
Building a list of nodes using the Views module, with identical selection criteria, is rather straight forward (not further explained here). And if you then use the Views Bulk Operations (VBO) module to transform that view into a VBO view, you can use a Rules Condition "*** Check number of results returned by a VBO View***" in which you can perfectly construct the Rules Condition you're looking for.
True, this requires 2 extra modules (which should be available in mostly any site anyway), but it requires zero custom code. And hence you avoid the need for the dangerous PHP filter.
PS: Have a look at "this answerthis answer" for an illustration of how this specific Rules Condition is used.
