What is the purpose of the "use" command and what is its relation to anonymous functions?

Question

I am doing some tests with a Restful API that I am creating using the Slim micro-framework, and in one of these routines that is responsible for executing a certain action caught my attention a command that I did not know, which is the command use.

See below for an example of the command use in a method that returns a JSON with the data obtained from the database:

//Listando todas pessoas
$app->get('/pessoas/', 
        function() use ($app) 
        {
            (new \controllers\Pessoa($app))->listar();            
        });

This command seems to be part of the anonymity function function() who is responsible for the route:

http://localhost/RestApiEx/pessoas/

And the use of it left me with the following doubts.

Doubts

I’d like to know what the purpose of command is use and what is the relationship that it has with Anonimas functions?

Answer 1

The use passes the value of one or more variables to the scope of the anonymity function making this value accessible (inheriting the variable), for example if you do this:

<?php

$a = 'Olá, mundo!';

$foo = function() {
   echo $a;
};

$foo();

You’ll find that the variable was not defined, something like:

PHP Notice: Undefined variable: a in example.php on line 6

But if I do this:

<?php

$a = 'Olá, mundo!';

$foo = function() use ($a) {
   echo $a;
};

$foo();

The value will be printed 'Olá, mundo!'

In the framework Slim the main variable of the application is the $app, but to make it accessible to the scope of the anonymity function use, if I did this:

$app->get('/pessoas/', 
        function() 
        {
            (new \controllers\Pessoa($app))->listar();            
        });

Would have a similar mistake:

PHP Notice: Undefined variable: app in exemplo.php on line 9

And your controller would get a value null instead of the object new \Slim\App;

Doc: http://php.net/manual/en/functions.anonymous.php

Extras

1. A detail when using use the variable $a example is not a reference, if you do something like:

   <?php
   
   $a = 1;
   
   $foo = function() use ($a) {
      $a += 10;
   };
   
   $foo();
   
   echo $a;
   

   Will be shown:

   1

   But if you make the reference:

   <?php
   
   $a = 1;
   
   $foo = function() use (&$a) {
      $a += 10;
   };
   
   $foo();
   
   echo $a;
   

   Will be shown 11:

   11

2. Something important to note is that in PHP there is another use, which is used to create nicknames for classes, thus:

   <?php
   
   use Foo\Bar\Baz; //é possivel chamar new Baz, sem o namespace completo
   use Foo\Bar\Baz as Test; //é possivel chamar new Test para se referir ao Foo\Bar\Baz
   

   Read more on /a/151492/3635