Which of the following is a valid PHP variable naming convention?

In PHP, variable names must adhere to specific rules and conventions for them to be valid. A valid variable name must start with a dollar sign ($) followed by a letter or an underscore, and it can contain letters, numbers, and underscores. It cannot start with a digit and cannot contain special characters such as hyphens.

The correct choice, which is represented by a variable name starting with a dollar sign followed by a sequence of valid characters, is optimal because it meets all the criteria set forth for naming a variable in PHP. In this case, $VariableName starts with a dollar sign, followed by an uppercase letter, which is acceptable in PHP naming conventions.

In contrast, the other options fail to comply with one or more of these rules. For example, a variable name that begins with a numerical digit or contains special characters like hyphens would not be recognized as valid in PHP. Thus, the selection of $VariableName embodies the proper convention for naming variables in PHP, ensuring that it functions correctly within the language syntax.