how to clear a field in database in x mins?

When you create the code and store it in the DB, also create a timestamp:

Code:

insert into whatever_table
(user_id, code, created_at)
values( ?, ?, NOW() );

Then when you want to check for a valid code:

Code:

select * from whatever_table
where user_id = ?
and code = ?
and created_at > ( NOW() - interval 1 hour );

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