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What is an anonymous function in PHP, and how does it differ from a regular named function? What is the use keyword in a PHP closure, and why is it needed to access outside variables from within an anonymous function? What are arrow functions introduced in PHP 7.4, and how do they simplify writing short closures compared to traditional anonymous functions? How can closures be used to implement simple dependency injection or callback based customization within a class? How does binding a closure to a specific object or class using bindTo or Closure::bind allow it to access private properties and methods it otherwise could not? How do first class callable syntax and higher order functions in modern PHP support building flexible, composable, functional style code?
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How does passing a variable by reference into a closure's use clause differ from the default pass by value behavior?

Intermediate

Answer

By default, variables imported using the use keyword are captured by value, meaning the closure receives a snapshot of that variable's value at the time the closure was defined, and any changes made to that variable later, either inside or outside the closure, do not affect the other's copy, but adding an ampersand before the variable name in the use clause captures it by reference instead, meaning changes made inside the closure directly affect the original outside variable, and vice versa.
$counter = 0;
$increment = function () use (&$counter) {
    $counter++;
};
$increment();
$increment();
echo $counter;
Real-world example A simple counter utility uses a closure that captures a shared counter variable by reference, allowing repeated calls to the closure to consistently increment the exact same underlying counter value across multiple invocations.

Common follow-ups: Why would a developer choose pass by reference over pass by value for a closure's use variable?;Are there any risks associated with capturing variables by reference in a closure?

Functions & Scope;OOP
All 7 Closures & Anonymous Functions questions