Assuming that the variable gets created in the scope of the function and that it never leaks outside of the scope (e.g. by reference), then you can consider these things:
PHP will destroy the object when there are no more references to it in the running script[1]. So, if the PDO object is limited to the function's scope, unsetting the variable, setting it to something else, or doing absolutely nothing will result in the same outcome. The code line `$stmt = null` doesn't directly affect the PDOStatement that was assigned to this variable previously. It just sets the value of the variable to something else. https://3v4l.org/VP5IJ Therefore, setting the variable to null as the last statement in the function is just adding an extra unnecessary line of code. I assume OPCache will optimize it away since it is dead code, but if not, then that's just one more operation PHP has to do as part of the function. >From my testing, it seems the assignment is not optimized away. So it's the opposite of an optimization. The `$stmt = null` is cargo-cult programming[2]. Most likely, the developer that came up with this guideline learned PHP years ago when mysql_* API was in use. After switching to PDO, they realized there is no close() method and, without understanding why, tried to come up with an alternate "solution". Or maybe they've run into an issue with out-of-sync operations on MySQL and saw setting the variable to null as a quick solution. Later, people kept copying this code without knowing why that line was there. Neither the connection nor the statement should be closed. An attempt to do so is a code smell that indicates that your code is poorly designed and needs to be refactored. In your example, the refactoring involves simply removing the dead-code assignment. In other cases, it could mean restricting the objects only to the relevant scope. In my opinion, there isn't any good reason to unset a variable. [1]: https://www.php.net/manual/en/language.oop5.decon.php#language.oop5.decon.destructor [2]: https://en.wikipedia.org/wiki/Cargo_cult_programming
