declare_vars examples

declare_vars.php

<?php

/*
 * When enabled, forces all variables to be declared before read/write access.
 * The feature can be turned on and off.
 */
declare(declare_vars=1);

var_dump($undeclaredVar);              // Compile error
$undeclaredVar = 'something';          // Compile error
isset($undeclaredVar);                 // Compile error

var $declaredVar;                      // Variable declared and can be used
var $initializedVar = 'Initial Value'; // Variable declared with initial value
var $object         = new stdClass();  // Any value allowed as initial value

var_dump($declaredVar);                // null
var_dump($initializedVar);             // "Initial Value"
var_dump($object);                     // Object(stdClass)

declare(declare_vars=0);

var_dump($anotherUndeclaredVar);       // No compile error

?>

<?php

/*
 * TBD: When disabled, the "var" keyword causes a compile error
 */

var $declaredVar;    // Compile error: declare_vars is disabled

?>

<?php

/*
 * Redeclaring variables is not allowed. This is necessary because variables may
 * be typed later.
 */

declare(declare_vars=1);

var $declaredVar;
var $declaredVar; // Compile error: variable already declared

?>

<?php

/*
 * Variables that are implicitly declared with declare_vars=0 count as declared
 * if declare_vars is turned on later in the file.
 */

$implicitlyDeclaredVar = 'Initial value'; // No compile error
var_dump($undeclaredVar);                 // No compile error

declare(declare_vars=1);

var_dump($implicitlyDeclaredVar);         // No error
var_dump($undeclaredVar);                 // Compile error

?>

<?php

// includedFile.php

var_dump($declaredVar);                    // null
var_dump($undeclaredVar);                  // Warning: undefined variable
$implicitlyDeclaredVar = 'Initial Value';  // Variable is implicitly declared

/*
 * When including files, declare_vars is not inherited. Scope, however, is.
 */

declare(declare_vars=1);

var $declaredVar;

include('includedFile.php');

var_dump($implicitlyDeclaredVar);          // "Initial Value"
var_dump($undeclaredVar);                  // Compile error

?>

<?php

/*
 * Variable variable access is evaluated at runtime. With declare_vars=1,
 * accessing an undeclared variable will throw an Error exception that can be
 * caught and handled.
 */

declare(declare_vars=1);

var $variableName = 'otherVariable';
var $otherVariable = 'foo';

var_dump($$variableName);              // "foo"
var_dump($$otherVariable);             // Error exception

?>

<?php

/*
 * It is possible to declare dynamic variables. However, they may not be
 * redeclared as per the example above.
 */

declare(declare_vars=1);

var $variableName = 'otherVariable';
var $$variableName = 'foo';

var_dump($otherVariable);      // "foo"

?>

<?php

/*
 * Variables remain declared when leaving a scope, even if the variable was
 * declared within that scope. A scope inherits all variables from the parent
 * scope.
 */

declare(declare_vars=1);

var $topScopedVar = 'foo';

{
    var $scopedVar = 'bar';

    var_dump($topScopedVar);    // "foo"
    var_dump($scopedVar);       // "bar"
}

var_dump($topScopedVar);        // "foo"
var_dump($scopedVar);           // "bar"

?>

<?php

/*
 * Classes declared within a declared_vars=1 scope require all properties to be
 * declared. This also counts if the code accessing a class instance is running
 * with declare_vars=0
 */

declare(declare_vars=1);

class DoesNotAllowUndeclaredProperties
{
    public $declaredProperty;

    public function __construct()
    {
        var $propertyName = 'undeclaredProperty';
        $this->undeclaredProperty = 'Initial value';      // Compile error
        $this->$propertyName = 'Initial Value';           // Error exception
    }
}

var $object = new DoesNotAllowUndeclaredProperties();
var $propertyName = 'undeclaredProperty';
var_dump($object->undeclaredProperty);                    // Compile error
var_dump($object->$propertyName);                         // Error exception

declare(declare_vars=0);

var_dump($object->undeclaredProperty); // Still a compile error

?>

<?php

/*
 * PHP's built-in classes are declared with declare_vars=0. This is especially
 * important for stdClass, which is used when returning objects from json_decode.
 */

declare(declare_vars=1);

$object = new stdClass();
$object->foo = 'bar';      // No error

?>

<?php

/*
 * TBD: Magic __get, __set, __isset, and __unset are not allowed when using
 * declare_vars=1
 */

declare(declare_vars=1);

// Compile error: using magic method "__get" is not allowed when using
// declare_vars=1

class AllowsUndeclaredPropertiesViaMagicMethods
{
    public function __get()
    {
        return 'foo';
    }

    public function __set()
    {
        // noop
    }
}

?>

<?php

declare(declare_vars=1);

/*
 * In the future, we may decide to implement lexical scope for variables. This
 * would undeclare all variables that were declared in the current scope once
 * leaving this scope
 */

declare(declare_vars=1);

var $topScopedVar = 'foo';

{
    var $scopedVar = 'bar';

    var_dump($topScopedVar);    // "foo"
    var_dump($scopedVar);       // "bar"
}

var_dump($topScopedVar);        // "foo"
var_dump($scopedVar);           // Compile error

?>

<?php

/*
 * Typed variables will require variable declarations
 */

declare(declare_vars=1);

var string $declaredString = 'Foo';
var int $declaredInt = 0;
var ?Class $object;

var_dump($object);                       // Uninitialized
$object = null;
var_dump($object);                       // null
$object = new stdClass();                // Type error
$object = new Class();
var_dump($object);                       // Object(Class)