Answer Posted / deepesh mehta

This type of variable diclearation is called "Variable
variables".
Sometimes it is convenient to be able to have variable
variable names. That is, a variable name which can be set
and used dynamically. A normal variable is set with a
statement such as:

$x = 'hello';

A variable variable takes the value of a variable and
treats that as the name of a variable. In the above
example, hello, can be used as the name of a variable by
using two dollar signs. i.e.

$$x = 'world';

At this point two variables have been defined and stored in
the PHP symbol tree: $a with contents "hello" and $hello
with contents "world". Therefore, this statement

echo "$x ${$x}";

produces the exact same output as:

echo "$a $hello";

i.e. they both produce: hello world.

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