, Such as the invalid value assignment method

For help, is this assignment method illegal? $ A & nbsp; $ a1 $ a2 $ a3 $ a4 $ a5; shared to: more ------ solution ----- help for help, is this assignment method illegal?
$ A = "$ a1
$ A2
$ A3
$ A4
$ A5
";

Share: More

------ Solution --------------------
It is valid if $ a1, $ a2, $ a3, $ a4, and $ a5 exist.
------ Solution --------------------
Naming rules for variables:

1. variable names are case sensitive.

2. the built-in structure, keywords, and user-defined class names and function names are case-insensitive.

3. it can only start with a letter or underline ..

Variable naming method:

Lowercase letters, expressive words, and underlines between words

Example: $ name $ post_form $ name_arr.