Detailed explanation of '$$' in $$str in php

This way of writing is called a variable variable

Sometimes it is convenient to use mutable variable names. That is, the variable name of a variable can be set and used dynamically. An ordinary variable is set by declaration, for example:

$a = "hello";
?>

A variable variable obtains the value of an ordinary variable as the variable name of the variable variable. In the above example, hello can be used as a variable variable after using two dollar signs ($). For example:

$$a = "world";
?>

At this point, both variables are defined: the content of $a is "hello" and the content of $hello is "world". Therefore, it can be expressed as:

echo "$a ${$a}";
?>

The following way of writing is more accurate and will output the same result:

echo "$a $hello";
?>

They will all output: hello world.

To use mutable variables with arrays, an ambiguity must be resolved. This is when writing $$a[1], the parser needs to know whether it wants $a[1] as a variable, or whether it wants $$a as a variable and extracts the variable with index [1] value. The syntax to solve this problem is to use ${$a[1]} for the first case and ${$a}[1] for the second case.

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php, str