PHP - $ and $$ Variables

PHP is a powerful scripting language created specifically for web development. One of its unique features is the use of variables. Variables in PHP begin with the dollar symbol ($). But there is another type of variable known as a $$ (double dollar sign) variable, which is a variable within itself.

In this chapter, we are going to learn about these variables through simple examples.

What is a $ Variable?

A $ variable is a normal PHP variable that contains a value. A variable can be allocated a wide range of values, including numbers, texts and arrays.

Example

Here is the basic usage of $ variable in which we have declared a variable $name and assign it the value. And with the help of echo, we print the value stored in $name −

<?php   
   $name = "Akshita";
   echo $name; 
   // Output: Akshita
?>

Normal vs Dynamic Variables

Normal Variable:The declaration of a normal variable is like this −

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

Dynamic Variable: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 ”$$” −

<?php     
   $$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 −

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

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

Output

Both produce the same output −

good morning
good morning

Example - Using Dynamic Variables

Take a look at this followingexample−

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

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

Output

It will produce the following output −

good morning
good morning

Example

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;  
?>

Output

Here, you will get the below outcome −

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

What is a $$ Variable?

A $$ variable is a variable within a variable. It means that the variable’s name is saved in another variable.

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 with Multiple $ Symbols

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;
?>

Output

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

$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 $$a[1], then the parser needs to know if you are refering to “$a[1]” as a variable or if you want “$$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";
?>

Output

It will produce the following output −

Intel
Linux
PHP
Intel
Linux
PHP

It may be noted that this technique cannot be used with PHP’s Super-global arrays (Several predefined variables in PHP are “superglobals”, which means they are available in all scopes throughout a script) within functions or class methods. The variable “$this” is a special variable in PHP and it cannot be referenced dynamically.