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The $var (single dollar) is a normal variable with the name var that stores any value like string, integer, float, etc.The $$var (double dollar) is a reference variable that stores the value of the $variable inside it.
To understand the difference better, let’s see some examples.
Example 1
<?php
$x = "abc";
$$x = 200;
echo $x."<br/>";
echo $$x."<br/>";
echo $abc;
?>
Output:
In the above example, we have assigned a value to the variable x as abc. Value of reference variable $$x is assigned as 200.
Now we have printed the values $x, $$x and $abc.
Example2
<?php
$x="U.P";
$$x="Lucknow";
echo $x. "<br>";
echo $$x. "<br>";
echo "Capital of $x is " . $$x;
?>
Output:
In the above example, we have assigned a value to the variable x as U.P. Value of reference variable $$x is assigned as Lucknow.
Now we have printed the values $x, $$x and a string.
Example3
<?php
$name="Cat";
${$name}="Dog";
${${$name}}="Monkey";
echo $name. "<br>";
echo ${$name}. "<br>";
echo $Cat. "<br>";
echo ${${$name}}. "<br>";
echo $Dog. "<br>";
?>
Output:
In the above example, we have assigned a value to the variable name Cat. Value of reference variable ${$name} is assigned as Dog and ${${$name}} as Monkey.
Now we have printed the values as $name, ${$name}, $Cat, ${${$name}} and $Dog.