PHP RFC: Fix inconsistent behavior of $this variable

• Version: 0.9
• Date: 2016-05-23
• Author: Dmitry Stogov, dmitry@zend.com
• Status: Accepted
• First Published at: https://wiki.php.net/rfc/this_var

Historically PHP implementation accessed special $this variable using two different methods. In some cases this might lead to significant inconsistency, when $this accessed through different methods might have different values.

class C {
  function foo() {
    var_dump($this);
  }
  function bar() {
    $a="this";
    $$a=42;
    var_dump($this); // prints int(42)
    $this->foo();    // prints object(C)#1 (0) {}
  }
}
$x = new C;
$x->bar();

This RFC proposes disabling modification of $this variable using “magic code” and make $this always behave in the same way.

The following code worked in PHP 7, but will emit compilation error in PHP 7.1

function foo($this) { // Fatal error: Cannot use $this as parameter
}

The following code leaded to “Cannot re-assign $this” compilation error. In PHP 7.1 it will produce more suitable error message - “Cannot use $this as static variable”.

static $this; // Fatal error: Cannot use $this as static variable

The following code worked in PHP 7, but will emit compilation error in PHP 7.1

global $this; // Fatal error: Cannot use $this as global variable

The following code worked in PHP 7, but will emit compilation error in PHP 7.1

try {
  ...
} catch (Exception $this) { // Fatal error: Cannot re-assign $this
}

The following code worked in PHP 7, but will emit compilation error in PHP 7.1

foreach ($a as $this) { // Fatal error: Cannot re-assign $this
}

It's not allowed to re-assign $this, so why it should be allowed to unset() it. The following code worked in PHP 7, but will emit compilation error in PHP 7.1

unset($this); // Fatal error: Cannot unset $this

An attempt to re-assign $this through $$ assignment will lead to throwing of Error exception.

$a = "this";
$$a = 42; // throw new Error("Cannot re-assign $this")

It's still possible to read $this value through $$.

Indirect re-assign $this through reference won't make effect in PHP 7.1

class C {
  function foo(){
    $a =& $this;
    $a = 42;
    var_dump($this); // prints object(C)#1 (0) {}, php-7.0 printed int(42)
  }
}
$x = new C;
$x->foo();

Few internal PHP functions may re-assign local variable. In PHP 7.1 they cannot change value of $this variable and throw Error exception.

class C {
  function foo(){
    extract(["this"=>42]);  // throw new Error("Cannot re-assign $this")
    var_dump($this);
  }
}
$x = new C;
$x->foo();

In PHP 7.0 and below get_defined_vars() might show or not show value of $this depending on some condition. (e.g. it was shown if we used $this variable itself, but not if it was used in a $this property reference or method call). In PHP 7.1 we won't show the value of $this in all cases.

In PHP 7.0 and below $this in static magic method __call() had value NULL. However it was possible to access properties and call object methods.

class C {
  static function __call($name, $args) {
    var_dump($this); // prints object(C)#1 (0) {}, php-7.0 printed NULL
    $this->test();   // prints "ops"
  }
  function test() {
    echo "ops\n"; 
  }
}
$x = new C;
$x->foo();

Attempt to use $this in plain function or method now will lead to exception “Using $this when not in object context”. This unifies behavior with method call and property access. Previously PHP emitted “undefined variable” notice and continued execution assuming $this is NULL. It's still possible to use isset($this) and empty($this) to check object context.

function foo() {
    var_dump($this); // throws "Using $this when not in object context"
                     // php-7.0 emitted "Undefined variable: this" and printed NULL
}
foo();

All the BC breaks are intentional and they are described in the proposal section.

PHP 7.1

none

none

The proposed implementation is compatible with opcache

none

The vote is a straight Yes/No vote, that requires a 2/3 majority. The voting began on Jun 6 and will close on Jun 16.

Fix inconsistent behavior of $this variable?
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Final result:	43	0
This poll has been closed.

PR 1918

After the project is implemented, this section should contain

1. the version(s) it was merged to
2. a link to the git commit a9512af8109e889eb2c6042c57797184930667cd
3. a link to the PHP manual entry for the feature