What is Closure::bind() in PHP

Question

The PHP manual offers little explanation about Closure::bind() and the example was confusing too.

Here's the code example on the site:

class A {
private static $sfoo = 1;
private $ifoo = 2;
}
$cl1 = static function() {
return A::$sfoo;
};
$cl2 = function() {
return $this->ifoo;
};

$bcl1 = Closure::bind($cl1, null, 'A');
$bcl2 = Closure::bind($cl2, new A(), 'A');
echo $bcl1(), "\n";
echo $bcl2(), "\n";

What are the parameters for Closure::bind()?

Null was used in the above, and even the "new" keyword was used too, which makes this even more confusing to me.

Answer 1

If you put the value of $cl2 as a method of class A, the class looks like this:

class A {
    public $ifoo = 2;

    function cl2()
    {
        return $this->ifoo;
    }
}

and you can use it like this:

$x = new A();
$x->cl2();
# it prints
2

But, because $cl2 is a closure and not a member of class A, the usage code above does not work.

The method Closure::bindTo() allows using the closure as it were a method of class A:

$cl2 = function() {
    return $this->ifoo;
};

$x = new A();
$cl3 = $cl2->bindTo($x);
echo $cl3();
# it prints 2

$x->ifoo = 4;
echo $cl3();
# it prints 4 now

The closure uses the value of $this but $this is not defined in $cl2. When $cl2() runs, $this is NULL and it triggers an error ("PHP Fatal error: Using $this when not in object context").

Closure::bindTo() creates a new closure but it "binds" the value of $this inside this new closure to the object it receives as its first argument.

Inside the code stored in $cl3, $this has the same value as the global variable $x. When $cl3() runs, $this->ifoo is the value of ifoo in object $x.

Closure::bind() is the static version of Closure::bindTo(). It has the same behaviour as Closure::bindTo() but requires an additional argument: the first argument must be the closure to bind.