What does a $$...

In PHP, the use of multiple dollar signs ($$) is known as a "variable variable". It is a way to create a variable whose name is determined by the value of another variable.

Here's how it works:

1. The first $ is used to define a variable, just like in a regular variable assignment.

2. The second $ is used to indicate that the value of the variable should be treated as a variable name.

3. If there are more than two $ signs (e.g., $$$), it means that the value of the variable is a variable that contains another variable name.

For example:

$foo = "bar";
$$foo = "baz";

echo $foo;    // Output: bar
echo $bar;    // Output: baz

In this example:

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