<?php
//You can even add more Dollar Signs
$Bar = "a";
$Foo = "Bar";
$World = "Foo";
$Hello = "World";
$a = "Hello";
$a; //Returns Hello
$$a; //Returns World
$$$a; //Returns Foo
$$$$a; //Returns Bar
$$$$$a; //Returns a
$$$$$$a; //Returns Hello
$$$$$$$a; //Returns World
//... and so on ...//
?>
가변 변수
때때로 가변 변수명을 갖을수 있는것다는 것은 편리함을 준다. 즉, 변수명이 유동적으로 설정되거나 사용될수 있다. 일반적인 변수는 다음과 같은 구문에 의해 설정된다:
<?php
$a = 'hello';
?>
가변변수는 변수값을 취해서 변수명으로 취급한다. 위 예제코드는, hello를 두개의 달러사인을 사용하여 변수명으로 사용할수 있다.
<?php
$$a = 'world';
?>
이 지점에서 두 변수가 선언되었고 PHP 심볼 트리에 저장된다: $a는 "hello" 값을 갖고 $hello는 "world" 값을 갖게 된다. 따라서, 이 구문:
<?php
echo "$a ${$a}";
?>
다음과 완전히 똑같이 출력된다:
<?php
echo "$a $hello";
?>
즉, 둘다 hello world를 출력한다.
배열을 갖는 가변변수를 사용하기 위해서는 애매한 문제를 해결해야 한다. 즉 $$a[1]를 쓴다면 해석기는 $a[1]가 변수를 의미하는지 알수 있어야 한다. 또는 $$a가 변수이기를 바라고, [1]가 그 변수의 인덱스인지 알수 있어야 한다. 이런 애매한 문제를 해결하기 위한 문법: 첫번째 목적을 위해 ${$a[1]}과 두번째 목적을 위해 ${$a}[1]을 들수 있다.
Warning
가변 변수로 함수나 클래스 메쏘드 안에서 PHP 자동 전역 배열을 사용할 수 없음에 주의하십시오. $this 변수도 특수 변수로써, 동적으로 참조할 수 없습니다.
add a note
User Contributed Notes 10 notes
userb at exampleb dot org ¶
14 years ago
marcin dot dzdza at gmail dot com ¶
5 years ago
The feature of variable variable names is welcome, but it should be avoided when possible. Modern IDE software fails to interpret such variables correctly, regular find/replace also fails. It's a kind of magic :) This may really make it hard to refactor code. Imagine you want to rename variable $username to $userName and try to find all occurrences of $username in code by checking "$userName". You may easily omit:
$a = 'username';
echo $$a;
sebastopolys at gmail dot com ¶
2 years ago
In addition, it is possible to use associative array to secure name of variables available to be used within a function (or class / not tested).
This way the variable variable feature is useful to validate variables; define, output and manage only within the function that receives as parameter
an associative array :
array('index'=>'value','index'=>'value');
index = reference to variable to be used within function
value = name of the variable to be used within function
<?php
$vars = ['id'=>'user_id','email'=>'user_email'];
validateVarsFunction($vars);
function validateVarsFunction($vars){
//$vars['id']=34; <- does not work
// define allowed variables
$user_id=21;
$user_email='email@mail.com';
echo $vars['id']; // prints name of variable: user_id
echo ${$vars['id']}; // prints 21
echo 'Email: '.${$vars['email']}; // print email@mail.com
// we don't have the name of the variables before declaring them inside the function
}
?>
Anonymous ¶
19 years ago
It may be worth specifically noting, if variable names follow some kind of "template," they can be referenced like this:
<?php
// Given these variables ...
$nameTypes = array("first", "last", "company");
$name_first = "John";
$name_last = "Doe";
$name_company = "PHP.net";
// Then this loop is ...
foreach($nameTypes as $type)
print ${"name_$type"} . "\n";
// ... equivalent to this print statement.
print "$name_first\n$name_last\n$name_company\n";
?>
This is apparent from the notes others have left, but is not explicitly stated.
jefrey.sobreira [at] gmail [dot] com ¶
9 years ago
If you want to use a variable value in part of the name of a variable variable (not the whole name itself), you can do like the following:
<?php
$price_for_monday = 10;
$price_for_tuesday = 20;
$price_for_wednesday = 30;
$today = 'tuesday';
$price_for_today = ${ 'price_for_' . $today};
echo $price_for_today; // will return 20
?>
Sinured ¶
17 years ago
One interesting thing I found out: You can concatenate variables and use spaces. Concatenating constants and function calls are also possible.
<?php
define('ONE', 1);
function one() {
return 1;
}
$one = 1;
${"foo$one"} = 'foo';
echo $foo1; // foo
${'foo' . ONE} = 'bar';
echo $foo1; // bar
${'foo' . one()} = 'baz';
echo $foo1; // baz
?>
This syntax doesn't work for functions:
<?php
$foo = 'info';
{"php$foo"}(); // Parse error
// You'll have to do:
$func = "php$foo";
$func();
?>
Note: Don't leave out the quotes on strings inside the curly braces, PHP won't handle that graciously.
mason ¶
14 years ago
PHP actually supports invoking a new instance of a class using a variable class name since at least version 5.2
<?php
class Foo {
public function hello() {
echo 'Hello world!';
}
}
$my_foo = 'Foo';
$a = new $my_foo();
$a->hello(); //prints 'Hello world!'
?>
Additionally, you can access static methods and properties using variable class names, but only since PHP 5.3
<?php
class Foo {
public static function hello() {
echo 'Hello world!';
}
}
$my_foo = 'Foo';
$my_foo::hello(); //prints 'Hello world!'
?>
herebepost (ta at ta) [iwonderr] gmail dot com ¶
8 years ago
While not relevant in everyday PHP programming, it seems to be possible to insert whitespace and comments between the dollar signs of a variable variable. All three comment styles work. This information becomes relevant when writing a parser, tokenizer or something else that operates on PHP syntax.
<?php
$foo = 'bar';
$
/*
I am complete legal and will compile without notices or error as a variable variable.
*/
$foo = 'magic';
echo $bar; // Outputs magic.
?>
Behaviour tested with PHP Version 5.6.19
Nathan Hammond ¶
16 years ago
These are the scenarios that you may run into trying to reference superglobals dynamically. Whether or not it works appears to be dependent upon the current scope.
<?php
$_POST['asdf'] = 'something';
function test() {
// NULL -- not what initially expected
$string = '_POST';
var_dump(${$string});
// Works as expected
var_dump(${'_POST'});
// Works as expected
global ${$string};
var_dump(${$string});
}
// Works as expected
$string = '_POST';
var_dump(${$string});
test();
?>
nils dot rocine at gmail dot com ¶
12 years ago
Variable Class Instantiation with Namespace Gotcha:
Say you have a class you'd like to instantiate via a variable (with a string value of the Class name)
<?php
class Foo
{
public function __construct()
{
echo "I'm a real class!" . PHP_EOL;
}
}
$class = 'Foo';
$instance = new $class;
?>
The above works fine UNLESS you are in a (defined) namespace. Then you must provide the full namespaced identifier of the class as shown below. This is the case EVEN THOUGH the instancing happens in the same namespace. Instancing a class normally (not through a variable) does not require the namespace. This seems to establish the pattern that if you are using an namespace and you have a class name in a string, you must provide the namespace with the class for the PHP engine to correctly resolve (other cases: class_exists(), interface_exists(), etc.)
<?php
namespace MyNamespace;
class Foo
{
public function __construct()
{
echo "I'm a real class!" . PHP_EOL;
}
}
$class = 'MyNamespace\Foo';
$instance = new $class;
?>
