#### Rules for PHP identifiers
1. A variable starts with the $ sign, followed by the name of the variable.
2. A variable name must start with a letter or the underscore character.
3. A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ ).
4. Identifiers are case sensitive. Therefore, a variable named $unicorn is different from a variable named $Unicorn, $uNicOrn, or $unicorN.
5. Identifiers can be any length.
6. An identifier name can’t be identical to any of PHP’s predefined keywords.