Explanation of '$$" in $$str in php, explanation of str in php_PHP tutorial

Explanation of "$$" in $$str in php, explanation of str in php

This way of writing is called a variable variable
Sometimes variable variable names are used It is very convenient. That is to say, the variable name of a variable can be set and used dynamically. An ordinary variable is set by declaration, for example:

$a = "hello";
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A mutable variable gets a The value of the ordinary variable is used as the variable name of this variable variable. In the above example, hello can be used as a variable variable after using two dollar signs ($). For example:

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

echo "$a ${$a}";
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The following writing is more accurate and will output the same The result:

echo "$a $hello";
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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.