Closures and capture policies

The idea

A PHP closure can import surrounding variables with use. A by-value capture records the variable’s value when the closure is created, while use (&$name) refers to the surrounding variable container and observes later changes. Arrow functions automatically capture used outer variables by value. Capture policy is therefore part of the behavior, not merely a syntactic preference.

By-value capture does not imply a deep copy of referenced objects. If the captured value is an object identifier, property mutation can still affect the shared object. A stateful closure can be useful for accumulating work, but a supposedly repeatable transformation should not depend on hidden changing state. Keep data ownership and lifetime visible when callbacks are stored and executed much later than their creation.

<?php
$n=2;
$value=function()use($n){return $n;};
$reference=function()use(&$n){return $n;};
$n=5;echo $value(),":",$reference(),"\n";

2:5

The value capture retains two, while the reference capture reads the current surrounding variable value five.