Why does PHP allow you to "declare" $this with Extract?

Question

I was doing some tests (playing) with the function extract and compact PHP. I was seeing how these two functions behaved with the variable $this.

Then I performed the following tests:

I tried to get the value:

var_dump($this); // Notice: Undefined variable 'this'

I tried to assign a value:

$this = 1; // Fatal Error:  Cannot re-assign $this

The above results is the obvious to expect. However when I used the function extract history has changed.

extract(['this' => 1]);

var_dump($this); // int(1)

Already within classes this does not happen.

I wonder if the above behavior with the function extract, unlike direct attribution, it is a common behavior or this is a bug.

The fact that language allows for this "attribution to $this" using extract may cause some problem?

Answer 1

Why this permission is really something that also intrigued me, is not considered bug, because it does not affect the functioning of language.

I tried to force one anyway bug or strange behavior, but as mentioned in the question, when used in class it is the only place in which a $this can be used, the code of the extract has no effect.

I used the method get_defined_vars() to check if a variable with a name is actually being declared $this and I noticed that you are declaring this variable when the extract is not within a non-static class, when it is within a non-static class it does nothing, because within the method extract there must be some control to it, and when it is in a static class the extract works, but it does not help a $this static class.

The most I could get was to take a copy of the class, when returning it to an external environment (outside the class) it is only possible to change and take public attributes, nor public methods can be called:

index php.:

require_once "TesteClass.php";
$teste = new TesteClass();
$teste->dump();
$retorno = $teste->ext();

echo "Retorno: ";
var_dump($retorno);
echo "<hr/>";
echo $retorno['this']->atributo."<br/>";
$retorno['this']->atributo = "Teste de troca de atributo fora da classe";

echo "Atributo da classe: " . $teste->getAtributo();
echo "<hr/>";

echo "Tentando acessar metodo public fora da classe (pela copia do this): ";
$retorno['this']->mPublico();
echo "<hr/>";

echo "Tentando acessar metodo privado fora da classe (pela copia do this): ";
$retorno['this']->mPrivado();
echo "<hr/>";

Testeclass.php:

<?php
class TesteClass
{
    public $atributo = "valor do meu atributo";

    public function getAtributo()
    {
        return $this->atributo;
    }

    public function dump()
    {
        echo "Minha classe: ";
        var_dump($this);
        echo "<hr/>";
    }

    public function ext()
    {
        extract(["classe" => get_defined_vars($this)]);     
        echo "Get Defined vars na classe: ";
        var_dump(get_defined_vars());
        echo "<hr/>";
        echo "Atributo do Extract : " . $classe['this']->atributo . "<br/>";
        $classe['this']->atributo = "Novo valor do atributo";
        echo "Novo Atributo do Extract : " . $classe['this']->atributo . "<br/>";
        echo "Novo Atributo do THIS : " . $this->atributo . "<br/>";
        echo "<hr/>";
        echo "Metodo privado chamadao da classe pela variavel do Extract : <br/>";
        $classe['this']->mPrivado();
        echo "<hr/>";
        return $classe;
    }

    private function mPrivado()
    {
        echo "Acessou o metodo privado!";
    }

    public function mPublic()
    {
        echo "Acessou o metodo publico!";
    }
}

The test will give a fatal error:

Fatal error: Cannot access private Property Testeclass::$attribute in C: wamp www test.php on line 11

As a conclusion it seems to me that it is more a forgetfulness of the developers than a bug itself, or else they saw that there is no possibility of an error arising from it, because they had already arranged so that when called by a non-static class the method did not create certain variables, and then let it occur.