PHP $ $str in "$$" Explanation of PHP in the str explanation _php Tutorial

PHP $ $str in the "$$" explanation, PHP in the str explanation

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

$a = "Hello";
?>

A mutable variable gets the value of an ordinary variable as the variable name of the variable variable. In the example above, hello uses two dollar sign ($) and can be used as a variable variable. For example:

$ $a = "world";
?>

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

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

The following wording is more accurate and will output the same result:

echo "$a $hello";
?>

They all output: Hello world.

To use mutable variables with arrays, you must address an ambiguous question. This is when you write the $ $a [1], the parser needs to know if you want to $a [1] as a variable, or you want $ $a as a variable and take out the value of the variable indexed to [1]. The syntax for solving this problem is to use ${$a [1] for the first case, and ${$a}[1 for the second case).

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