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What is an anonymous function in PHP, and how does it differ from a regular named function? How does passing a variable by reference into a closure's use clause differ from the default pass by value behavior? 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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What is the use keyword in a PHP closure, and why is it needed to access outside variables from within an anonymous function?

Beginner

Answer

By default, an anonymous function in PHP does not have access to variables from the surrounding scope where it was defined, so the use keyword explicitly lets you import specific outside variables into the closure, making them available inside the anonymous function's own body, which is necessary because closures otherwise operate in an isolated scope for clarity and to avoid unintended side effects.
$taxRate = 0.08;
$calculateTax = function ($price) use ($taxRate) {
    return $price * $taxRate;
};
echo $calculateTax(100);
Real-world example A pricing calculation function uses the use keyword to bring the current tax rate into a closure responsible for calculating tax on individual product prices, keeping that shared value accessible without making it a global variable.

Common follow-ups: What is the difference between passing a variable into use by value versus by reference?;What happens if you modify a use variable inside the closure without passing it by reference?

Functions & Scope;Basics & Types
All 7 Closures & Anonymous Functions questions