Closures in PHP… what, precisely, are they and when would you need to use them?

Question

So I\'m programming along in a nice, up to date, object oriented fashion. I regularly make use of the various aspects of OOP that PHP implements but I am wondering when might I

Answer 1

Closures:

MDN has the best explanation IMO:

A closure is the combination of a function bundled together (enclosed) with references to its surrounding state (the lexical environment). In other words, a closure gives you access to an outer function’s scope from an inner function.

i.e. A closure is a function with access to the variables which are in the parent scope. A closure allows us to conveniently create functions on the fly since in some situations a functions is only needed in one place (callbacks, callable arguments).

Example:

$arr = [1,2,3,3];
$outersScopeNr = 2;

// The second arg in array_filter is a closure
// It would be inconvenient to have this function in global namespace
// The use keyword lets us access a variable in an outer scope
$newArr = array_filter($arr, function ($el) use ($outersScopeNr) {
    return $el === 3 || $el === $outersScopeNr;
});

var_dump($newArr);
// array (size=3)
//  1 => int 2
//  2 => int 3
//  3 => int 3