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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? 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 an anonymous function in PHP, and how does it differ from a regular named function?

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Answer

An anonymous function, also called a closure, is a function without a name that can be assigned to a variable, passed as an argument to another function, or returned from a function, which is especially useful for short pieces of logic used only in one specific place, unlike a regular named function which is defined once and referenced by that name everywhere it is needed throughout your code.
$greet = function ($name) {
    return "Hello, $name!";
};
echo $greet('Ali');
Real-world example A form validation system uses a short anonymous function to define a one off custom validation rule for a specific field, avoiding the need to create and name a separate standalone function that would only ever be used in that single spot.

Common follow-ups: When should you use a named function instead of an anonymous function?;Can an anonymous function be assigned to a class property?

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All 7 Closures & Anonymous Functions questions