Hidden trouble with PHP double quotes

PHP has a lot of syntax features that can be exploited by attackers, such as PHP to detect variables in double quotes.
Execute the following code:

1 function Test () 2 {3echo "abc"; 4 }5echo "${@test ()}"; 6 7 // or 8 Echo "${@phpinfo ()}";

The principle is as follows:

1 $a = ' B '; 2 $b = ' a '; 3 4 Echo $$a//a

The above takes advantage of PHP mutable variables, double quotes {} can parse the variable content characteristics inside the double quotes to create a little trouble.

Hidden trouble with PHP double quotes