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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