What does $$ (dollar dollar or double dollar) means in PHP ?

The $x (single dollar) is the normal variable with the name x that stores any value like string, integer, float, etc. The $$x (double dollar) is a reference variable that stores the value which can be accessed by using the $ symbol before the $x value. These are called variable variables in PHP.

Variable Variables:- Variable variables are simply variables whose names are dynamically created by another variable’s value.

 From the given figure below, it can be easily understood that:

• $x stores the value “Geeks” of String type.
• Now reference variable $$x stores the value “for Geeks” in it of String type.

So, the value of “for geeks” can be accessed in two ways which are listed below:

• By using the Reference variable directly. Example: echo $$x;
• By using the value stored at variable $x as a variable name for accessing the “for Geeks” value. Example: echo $Geeks; which will give output as “for Geeks” (without quote marks).

Examples:

Input : $x = "Geeks";  
        $$x = for Geeks;  
        echo "$x ";  
        echo "$$x;";   
        echo $Geeks;
Output : Geeks 
         for Geeks
         for Geeks

Input : $x = "Rajnish";  
        $$x = "Noida";  
        echo "$x lives in " . $Rajnish;
Output : Rajnish lives in Noida

Below are examples illustrating the use of $ and $$ in PHP: 

Example-1:

php

<?php

// Declare variable and initialize it
$x = "Geeks";

// Reference variable
$$x = "GeeksforGeeks";

// Display value of x
echo $x . "\n";

// Display value of $$x ($Geeks)
echo $$x . "\n";

// Display value of $Geeks
echo "$Geeks";

?>

Output:

Geeks
GeeksforGeeks
GeeksforGeeks

Example-2:

php

<?php

// Declare variable and initialize it
$var = "Geeks";

// Reference variable
${$var}="GeeksforGeeks";

// Use double reference variable
${${$var}}="computer science";

// Display the value of variable
echo $var . "\n";
echo $Geeks . "\n";
echo $GeeksforGeeks . "\n";

// Double reference
echo ${${$var}}. "\n";

?>

Output:

Geeks
GeeksforGeeks
computer science
computer science