PHP - $ and $$ Variables

Introduction

PHP uses the convention of prefixing the variable names by the $ symbol. PHP also has the provision of declaring dynamic variables by prefixing two dollar symbols $$ to the name. A variable variable (or a dynamic variable) can be set and used dynamically.

Syntax

The declaration of a normal variable is like this −

$a = 'good';

A dynamic variable takes the value of a normal variable and treats that as the name of the variable. In the above example, "good" can be used as the name of a variable by using two dollar signs $$ −

$$a = 'morning';

We now have two variables: $a with contents good and $$a with contents morning. As a result, the following echo statements will produce the same output −

echo "$a {$$a}";
echo "$a $good";

Both produce the same output −

good morning

Example 1

Take a look at this following example −

<?php  
   $a = 'good';
   $$a = 'morning';

   echo "$a {$$a}\n";
   echo "$a $good";
?>

It will produce the following output −

good morning
good morning

Example 2

Let's take a look at another example −

<?php  
   $x = "foo";  
   $$x = "bar";  
   echo "Value of x = " .$x . "\n";  
   echo 'Value of $$x = ' . $$x . "\n";  
   echo 'Value of foo = ' . $foo;  
?>

Here, you will get the following output −

Value of x = foo
Value of $$x = bar
Value of foo = bar

Using Multiple $ Symbols

Note that the use of $ symbol is not restricted to two. Any number of dollar symbols can be prefixed.

Suppose there is a variable $x with a as its value. Next, we define $$x='as', then $$x as well as $a will have the same value. Similarly, the statement $$$x='and' effectively declares a $as variable whose value is 'and'.

Example

Here is a complete example that shows the use of multiple $ symbols.

<?php  
   $php = "a";
   $lang = "php";
   $World = "lang";
   $Hello = "World";
   $a = "Hello";
   echo '$a= ' . $a;
   echo "\n";
   echo '$$a= ' . $$a;
   echo "\n";
   echo '$$$a= ' . $$$a;
   echo "\n";
   echo '$$$$a= ' . $$$$a;
   echo "\n";
   echo '$$$$$a= ' . $$$$$a;
?>

When you run this code, it will produce the following output −

$a= Hello
$$a= World
$$$a= lang
$$$$a= php
$$$$$a= a

Using Dynamic Variables with Arrays

Using dynamic variables with arrays may lead to certain ambiguous situations. With an array a, if you write $$a[1], then the parser needs to know if you are referring to $a[1] as a variable or if you want $$a as the variable and then the [1] index from that variable.

To resolve this ambiguity, use ${$a[1]} for the first case and ${$a}[1] for the second.

Example

Take a look at the following example −

<?php  
   $vars = array("hw", "os", "lang");
   $var_hw="Intel";
   $var_lang="PHP";
   $var_os="Linux";

   foreach ($vars as $var)
      echo ${"var_$var"} . "\n";

   print "$var_hw\n$var_os\n$var_lang";
?>

It will produce the following output −

Intel
Linux
PHP
Intel
Linux
PHP

note

This technique cannot be used with PHP's Superglobal arrays within functions or class methods. The variable $this is a special variable in PHP and it cannot be referenced dynamically.