HELP Tuning SQL statements

When I use the EXPLAIN PLAN to view the execution plan for my statement. The optimizer ignores my index for:

1. Select * from employees where employee_id = uid; uid is a variable from my PL/SQL procedure. The Explain show a full table access even when I use Hints. However,
2. Select * from employees where employee_id = '12345'; will access the table by rowid/ unique index scan on employee_id.

I'm trying to tune an Oracle Web application, and I'm not sure how to use bind variable (eg. :uid). Is there a way to use index for my first statement.

Thanx

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