I thought this stored procedure would be faster than plain selects

Could be the SELECT * try it with the column names instead and see if it speeds up.

Indeed, the following in particular looks bad:

Code:

SELECT * FROM categories WHERE 1 = 0

Because you're doing a SELECT on a table, even it is obvious that no data would actually be selected (as nothing will satisfy 1 = 0), this likely results in a rowscan of the entire table.

And of course, if your PHP solution doesn't involve a temporary table, that's another thing that adds overhead.