PHP Learning notes-mutable variables

See an example.

<?php$a = ' Hello '; $hello = ' Hi '; echo $ $a;? >

If a variable name is preceded by two dollar signs, then this variable is called a mutable variable. Take the above example, the variable variable means to find the value of the variable $ A, it is hello, and then the value as a variable name to form a new variable,

Is $hello, and then find the value of this $hello variable, it is Hi, so the value of $ $a is hi

Variable variable is the peeling from inside out, then the value of each layer of skin as the variable name to find the lower skin. In fact, it is similar to the equivalent in mathematics.

PHP Learning notes-mutable variables