In this new post to the PHP.cc blog, Stefan Priebsch looks at the relationship between urban legends and error handing in PHP.
Ending up in such a situation [as the tourists in the urban legend] is not the fault of the underpass: the error, made somewhere upstream, merely shows up right when approaching the underpass, and is usually difficult (or at least tedious) to handle. PHP as a programming language, just like any dynamic language, may at first appear to not need overly strict validations of data. Validation of data, however, is directly related to error handling: If data is invalid, this should be an error or an exception (if you write OOP code, which you should be doing). This error needs to be handled somewhere.
He talks about how things like field and value validation can help prevent errors further down the road. He mentions the split between application and domain logic and suggests that it's the role of the app logic to prevent bad data from making it thorough to the domain.
In software development, it is crucial that you handle any errors as soon as they occur. This requires you to actually realize that an error has occurred.