变量返回全文搜索的所有结果？

This is my first time using the full text search feature in MySQL.

I am using PDO statements and a select * statement.

SELECT *   
FROM `table` 
WHERE MATCH (var1) AGAINST (:field1 IN BOOLEAN MODE)
  AND MATCH (var2) AGAINST (:field2 IN BOOLEAN MODE)

If var1 or var2 are undefined, what variable can I set them to so that all results from field1 or field2 are displayed? Currently I get no results.

Before using Full Text search I would use

if (!isset($var1)) {
    $var1 = '%' . '' . '%';
}

with a LIKE statement that would return all results when a user has not specified any selection, etc.

I have tried using * but this doesn't work either.

Thanks