controlling user input.

[Anon]

I have some javascript tests online and will be collecting the answers with PHP and mySQL. Is there a way to set up the answers table so it can only be updated once per user? In other words, after the user takes the test and hits the submit button I want the database to close for his username and password. That way the user can only take the test once and not go look at the answers and change the test to all of the correct answers.

[Anon]

Obviously you must already have some way to identify users
uniquely (e.g. email address, etc.) Although you say nothing
about the database schema you are using, conceptually there
is nothing hard about enforcing a UNIQUE constraint on the
user ID in a "master" table. Having done this, the only thing
left to do is to detect the database error when you try to
insert a conflicting row, and writing some kind of meaningful
error message.

Using PostgreSQL for example:

create table test ( user_id text unique, test_id text );

Now you can say:

pg_exec($conn, 'begin' );
// must do this inside a transaction!

if (!($result = @pg_exec ($conn,
   "insert into test values ( 'me_again', 'php_database' ) ")))
  {
  if ( strpos( $php_errormsg,
    'Cannot insert a duplicate key' ) > 0)
      {
      echo "Sorry, you've already taken this test!"