PHP-SQL : What is the solution if you want to do a search and use UNION with two tables that have differents columns?

PHP-SQL : What is the solution if you want to do a search (with match and against) and use UNION with two tables that have differents columns (numbers, types, names) because if you use UNION, the columns have to have same name, number and type? All I found was to use NULL and ALIAS to simulate the columns correponding. Is there another solution for this case ?