What happens when you coerce a string to a date-and-time but it’s not valid?
Where I’m from (Rust), error handling is very strict and very explicit, and that’s how it should be. It forces you to properly handle everything that can potentially go wrong, instead of just crashing and looking like a fool.
My point is, you won’t ever try. You’d only use “weak” variables inside the function you’re working on.
It’s explicit when you absolutely need it to be, when the function is being called and you need to know what arguments to pass and what it’ll return
A string being parsed as a date-time is presumably user input, which is potentially invalid.
When you say user, you mean a user of a function? In that case PHP would throw a TypeError, and presumably only happens when developing/testing.
If you mean in production, like when submitting a form, an Exception may be thrown. In which case you catch it and return some error message to the user saying the date string is invalid.