Fix inconsistent behavior of $this variable View as Markdown

Introduction

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.

Proposal

Disable using $this as parameter

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
}

Disable using $this as static variable

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

Disable using $this as global 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

Disable using $this as catch 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
}

Disable using $this as foreach value variable

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
}

Disable ability to unset() $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

Disable ability to re-assign $this indirectly through $$

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 $$.

Disable ability to re-assign $this indirectly through reference

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();

Disable ability to re-assign $this indirectly through extract() and parse_str()

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();

get_defined_vars() always doesn't show value of variable $this

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.

Always show true $this value in magic method __call()

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();

Using $this when not in object context

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();

Backward Incompatible Changes

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

Proposed PHP Version(s)

PHP 7.1

RFC Impact

To SAPIs

none

To Existing Extensions

none

To Opcache

The proposed implementation is compatible with opcache

Open Issues

none

Proposed Voting Choices

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.

Vote: Fix inconsistent behavior of $this variable? — Yes: 43 · No: 0 (closed)

Patches and Tests

PR 1918

Implementation

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