PHP - $ and $$ Variables

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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 following example −

 <?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.