What are the rules for creating valid variable names in PHP according to the PHP manual?

In PHP, variable names must start with a letter or underscore, followed by any number of letters, numbers, or underscores. Variable names are case-sensitive, so $myVar and $MyVar are considered different variables. It's important to follow these rules when naming variables in PHP to avoid syntax errors and ensure code readability.

// Valid variable names in PHP
$myVar = "Hello";
$_myVar = "World";
$my_Var = "!";