This is possible, because what gets bound is a reference to the variable in question, but I find it horribly bad style:
It makes code harder to read, maintain and debug - the variable could be changed further down the line, or even in other functions called in between the binding and the query.
Binding a variable before declaring it will throw an E_NOTICE
message (No it doesn't. Cheers @webbiedave)
If you ask me, a query should be built in one place, and then executed straight away, for the sake of future readability.
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