In PHP, a function is defined with the function keyword, followed by a unique function name and a pair of _______ containing optional parameters.

The correct option is: "parentheses." In PHP, a function is defined using the function keyword, followed by the function name and a pair of parentheses. Within the parentheses, parameters can be defined to accept inputs for the function. Learn more: https://www.php.net/manual/en/functions.user-defined.php