when you run a query, you should check to see if it resulted in an error and then use the error retrieval function to tell you what went wrong. the error message usually tells you if you have a syntax error.
also, we haven't seen the actual query that gets run -- just the PHP statement that is defining the query. if $reminder is non-numeric then you'll get a syntax error. you might want to put quotes around $reminder.
additionally, we don't know if you are using single quotes or double quotes in this php statement. If you delimit the query string with single quotes, then $user_email and $reminder will not be evaluated to get the contents of those variables, your query will simply put the variables names in their -- dollar sign and all -- rather than evaluating them.
lastly, your post hardly gives any info about what's happening and you don't use the code formatting tags. you'll get answers faster if you learn to provide all the information needed to solve your problem.