PHP Functional programming edit

Closures

A closure is an anonymous function that can’t access outside scope.

When defining an anonymous function as such, you’re creating a “namespace” for that function. It currently only has access to that namespace.

$externalVariable = "Hello";
$secondExternalVariable = "Foo"; 
$myFunction = function() { 
  
  var_dump($externalVariable, $secondExternalVariable); // returns two error notice, since the variables aren´t defined 

}

It doesn’t have access to any external variables. To grant this permission for this namespace to access external variables, you need to introduce it via closures (use()).

$myFunction = function() use($externalVariable, $secondExternalVariable) {
   var_dump($externalVariable, $secondExternalVariable); // Hello Foo
}

This is heavily attributed to PHP’s tight variable scoping - If a variable isn’t defined within the scope, or isn’t brought in with global then it does not exist.

Also note:

Inheriting variables from the parent scope is not the same as using global variables. Global variables exist in the global scope, which is the same no matter what function is executing. The parent scope of a closure is the function in which the closure was declared (not necessarily the function it was called from).

Taken from the PHP Documentation for Anonymous Functions


In PHP, closures use an early-binding approach. This means that variables passed to the closure’s namespace using use keyword will have the same values when the closure was defined.

To change this behavior you should pass the variable by-reference.

$rate = .05;

// Exports variable to closure's scope
$calculateTax = function ($value) use ($rate) {
    return $value * $rate;
};

$rate = .1; 

print $calculateTax(100); // 5
$rate = .05;

// Exports variable to closure's scope
$calculateTax = function ($value) use (&$rate) { // notice the & before $rate
    return $value * $rate;
};

$rate = .1;

print $calculateTax(100); // 10

Default arguments are not implicitly required when defining anonymous functions with/without closures.

$message = 'Im yelling at you';

$yell = function() use($message) {
    echo strtoupper($message);
};

$yell(); // returns: IM YELLING AT YOU

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