Php variable naming rules
• Variable names must start with a letter of the alphabet or the _ (underscore) character.
• Variable names can contain only the characters: a-z, A-Z, 0-9, and _ (underscore).
• Variable names may not contain spaces. If a variable must comprise more than one word it should be separated with the _ (underscore) character. (e.g., $user_name).
• Variable names are case-sensitive. The variable $High_Score is not the same as the variable $high_score.