A callback is the process of calling a callable (i.e. function, method, or closure) to execute (i.e. invoking it) by passing its fully qualified name into a function. Huh?
Normally you call a function directly by its name and then put parentheses at the end of it. With a callback, you are calling it by passing its name as a string to a function. Then that function calls it and passes any arguments to it.
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