Starting PHP 5.4+, one of the uses of the static
keyword in PHP is to define static anonymous functions / closures. Read below to understand what they are and how they differ from non-static anonymous functions / closures.
Non-static Anonymous Functions
Before we dive into static anonymous functions, let's quickly go over the behavior of non-static anonymous functions.
When a closure is declared within the context of a class, the default behavior is that the class is automatically bound to the closure. What that means is that $this
is available inside of the anonymous function's scope. Let's consider the following example:
class Foo { public function bar() { return function() { var_dump($this); }; } } $foo = new Foo(); $func = $foo->bar(); $func();
The code above would output: object(Foo)#1 (0) { }
. From the result we can see how $this
refers to the class the closure was defined in.
Static Anonymous Functions
Simply put, static anonymous functions prevent:
- The automatic binding of a class to the closure defined within the its context;
- Binding of objects to the closure at runtime.
To illustrate this, let's consider the following examples:
class Foo { public function bar() { return static function() { var_dump($this); }; } } $foo = new Foo(); $func = $foo->bar(); $func();
This would result in: Notice: Undefined variable: this in %s on line %d NULL
.
Now let's see what happens when we try to bind an object to the anonymous function at runtime:
$func = static function() { // ... }; $func = $func->bindTo(new StdClass()); $func();
This would result in: Warning: Cannot bind an instance to a static closure in %s on line %d
.
When to Use Static Closures?
In general, static closures provide micro-optimization (for e.g., by improving hydration performance when dealing with private properties). You should consider using static closures when:
- You are certain you won't need to reference the class within the function scope;
- You don't have the need to bind an object to the function scope at runtime;
- You are looking for areas to optimize your code.
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