是否可以在PHP 5.2.x中模拟闭包而不使用全局变量？

Question

Is it possible to simulate closures in PHP 5.2.x not using globals? I could think of a way that would pass the desired variables as extra parameters to the closure but that just doesn't feel like best practice.

Any ideas?

Answer 1

Interesting question. I'd say it's not possible at all, but let's see

Quoting IBM - What's new in PHP5.3, Part 2

A closure is a function that is evaluated in its own environment, which has one or more bound variables that can be accessed when the function is called.

and further (emphasis mine)

Variables to be imported from the outside environment are specified in the use clause of the closure function definition. By default, they are passed by value, meaning that if we would update the value passed within the closure function definition, it would not update the outside value.

Using global would pass by reference and although it is possible to bind variables by reference with a closure by using & in the use clause, it is already a deviation from the 5.3 default behavior.

$var = 'yes';
$fn  = create_function('', 'global $var; $var = "no";');
$fn();
echo $var; // outputs no

You could copy the global variable in order to use it by value though, e.g.

$var = 'yes';
$fn  = create_function('', 'global $var; $tmp = $var; $tmp = "no";');
$fn();
echo $var; // outputs yes

In addition, the value of the global variable (when using create_function) will not be evaluated (bound) when the function is created but when the function is run

$var = 'yes';
$fn  = create_function('', 'global $var; $tmp = $var; return $tmp;');
$var = 'maybe';
echo $fn(); // outputs maybe

$var = 'yes';
$fn  = function() use ($var) { return $var; };
$var = 'maybe';
echo $fn(); // outputs yes

Also important is

When defined within an object, one handy thing is that the closure has full access to the object through the $this variable, without the need to import it explicitly. ^{*Though I think this was dropped in final PHP5.3}

This is impossible with the global keyword and you also cannot just use $this. There is no way to reference a property from a class when defining the function body with create_function.

class A {

    protected $prop = 'it works';

    public function test()
    {
        $fn = create_function('', 'echo $this->prop;');
        return $fn;
    }
}

$a = new A;
$fn = $a->test();
$fn();

will result in

Fatal error: Using $this when not in object context

To sum this up
While you can create a function importing a variable from the global scope, you cannot cannot create one using variables from another scope. And because you are technically not binding when using create_function but importing when the created function is executed, I'd like to argue this limitation makes the closure a lambda.

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EDIT: The solution offered by Onno Marsman below is pretty decent though. It doesn't fully simulate Closures, but the implementation is pretty close.