Closures in PHP… what, precisely, are they and when would you need to use them?

Question

So I\'m programming along in a nice, up to date, object oriented fashion. I regularly make use of the various aspects of OOP that PHP implements but I am wondering when might I

Answer 1

PHP will support closures natively in 5.3. A closure is good when you want a local function that's only used for some small, specific purpose. The RFC for closures give a good example:

function replace_spaces ($text) {
    $replacement = function ($matches) {
        return str_replace ($matches[1], ' ', ' ').' ';
    };
    return preg_replace_callback ('/( +) /', $replacement, $text);
}

This lets you define the replacement function locally inside replace_spaces(), so that it's not:
1) Cluttering up the global namespace
2) Making people three years down the line wonder why there's a function defined globally that's only used inside one other function

It keeps things organized. Notice how the function itself has no name, it simply is defined and assigned as a reference to $replacement.

But remember, you have to wait for PHP 5.3 :)

You can also access variables outside it's scope into a closure using the keyword use. Consider this example.

// Set a multiplier  
 $multiplier = 3;

// Create a list of numbers  
 $numbers = array(1,2,3,4);

// Use array_walk to iterate  
 // through the list and multiply  
 array_walk($numbers, function($number) use($multiplier){  
 echo $number * $multiplier;  
 }); 

An excellent explanation is given here What are php lambdas and closures