匿名函數（閉包）是PHP中沒有名稱的函數，常用於需要臨時定義回調函數的場景。它們可以賦值給變量或直接作為參數傳遞，常見於array_map、array_filter等數組操作及事件處理中。使用use關鍵字可讓閉包繼承父作用域中的變量，默認按值傳遞，若需修改外部變量則應使用&符號按引用傳遞。常見應用場景包括：1. 數組處理；2. 事件註冊；3. 維護狀態(tài)的回調；4. 自定義排序邏輯。閉包有助於保持代碼簡潔，但需注意變量作用域與傳遞方式。

Anonymous functions, often called closures in PHP, are functions without a name. They're super handy when you need a quick function for a specific task and don't want to define a full named function. You'll often see them used as callbacks, especially in array functions like array_map or array_filter , or when working with objects that require callback handlers.

What Exactly Is an Anonymous Function?

An anonymous function is just like a regular function, but it doesn't have a name. You can assign it to a variable or pass it directly as an argument. Here's a basic example:

 $greet = function($name) {
    echo "Hello, $name!";
};

$greet("John"); // Outputs: Hello, John!

In this case, the function is stored in the $greet variable and then called like a regular function. This pattern is common in PHP when you only need a function once or when creating closures that capture external variables.

Why Use the use Keyword?

When you define an anonymous function, it doesn't automatically have access to variables from the parent scope. That's where the use keyword comes in. It allows the closure to inherit variables from the surrounding code.

Here's how it works:

 $name = "Alice";

$greet = function() use ($name) {
    echo "Hello, $name";
};

$greet(); // Outputs: Hello, Alice

Without use , the function wouldn't know what $name is.

A few important things to note:

• Variables passed with use are inherited by value by default (they're copies).
• If you want to modify the original variable inside the closure, you need to pass it by reference using & .

Example of passing by reference:

 $count = 0;

$increment = function() use (&$count) {
    $count ;
};

$increment();
echo $count; // Outputs: 1

This behavior is crucial when you're building stateful closures or need to track changes outside the function itself.

Common Use Cases

You'll typically see closures and the use keyword in these scenarios:

• Array Functions : When filtering or mapping arrays.

   $numbers = [1, 2, 3, 4];
  $threshold = 2;
  $filtered = array_filter($numbers, function($n) use ($threshold) {
      return $n > $threshold;
  });

• Event Handlers : In frameworks or libraries where callbacks are registered dynamically.

• Stateful Callbacks : When maintaining some context across multiple calls without using global variables.

• Creating Custom Sort Logic :

   usort($users, function($a, $b) use ($sortBy) {
      return strcmp($a->$sortBy, $b->$sortBy);
  });

  These examples show how closures help keep your code clean and focused on the task at hand.

  ---

  So yeah, closures are a powerful feature in PHP, especially when combined with the use keyword. They let you write concise, localized logic without polluting the global namespace. Just remember: if you need external variables, always reach for use , and be mindful of whether you're passing by value or reference.

  基本上就這些。

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