Question

What are the differences between closures in JS and closures in PHP? Do they pretty much work the same way? Are there any caveats to be aware of when writing closures in PHP?

Answer 1

One difference is how both cope with storing the context in which an anonymous function is executed:

// JavaScript:
var a = 1;
var f = function() {
   console.log(a);
};
a = 2;
f();
// will echo 2;

// PHP
$a = 1;
$f = function() {
    echo $a;
};
$a = 2;
$f();
// will result in a "PHP Notice:  Undefined variable: a in Untitled.php on line 5"

To fix this notice you’ll have to use the use syntax:

$a = 1;
$f = function() use ($a) {
    echo $a;
};
$a = 2;
$f();
// but this will echo 1 instead of 2 (like JavaScript)

To have the anonymous function behave somehow like the JavaScript counterpart you’ll have to use references:

$a = 1;
$f = function() use (&$a) {
    echo $a;
};
$a = 2;
$f();
// will echo 2

I think this is the most striking difference between JavaScript and PHP closures.

Second difference is that every JavaScript closure has a this context available which means, that you can use this inside the closure itself (although it’s often quite complicated to figure out what this actually refers to) – PHP’s current stable version (PHP 5.3) does not yet support $this inside a closure, but PHP’s upcoming version (PHP 5.4) will support $this binding and rebinding using $closure->bind($this) (See the Object Extension RFC for more info.)

Third difference is how both languages treat closures assigned to object properties:

// JavaScript
var a = {
    b: function() {}
};
a.b(); // works


// PHP
$a = new stdClass();
$a->b = function() {};
$a->b(); // does not work "PHP Fatal error:  Call to undefined method stdClass::b() in Untitled.php on line 4"

$f = $a->b;
$f(); // works though

The same is true if closures are assigned to properties in class definitions:

class A {
    public $b;

    public function __construct() {
        $this->b = function() {};
    }

    public function c() {
        $this->b();
    }
}
$a = new A();
// neither
$a->b();
// nor
$a->c();
// do work

Fourth difference: JavaScript Closures are full fledged objects, wheres in PHP they are restricted objects. For instance, PHP Closures cannot have properties of their own:

$fn = function() {};
$fn->foo = 1;
// -> Catchable fatal error: Closure object cannot have properties

while in JavaScript you can do:

var fn = function() {};
fn.foo = 1;
fn.foo; // 1

Fifth difference: Returned closures can be immediately called upon in Javascript:

var fn = function() { return function() { alert('Hi');}}
fn()();    

Not in PHP:

$fn = function() { return function() { echo('Hi');};};
$fn()();     // syntax error