Error reporting

I'm working with PHP and Postgres.

I want to start making more use of the database's ability to maintain the
integrity of data. For example, when adding a value to a table where I
don't want duplicates. Instead of pre searching with PHP for the submitted
value in the table, then if the value isn't already existing in the table,
submit the new value - I was just going to add the new value, then check
the contents of pg_errormessage() to determine the success or failure of
the command.

Pg_errormessage returns a nice long string about there not being allowed
duplicate values. This isn't the easiest return for handling error
checking, ideally I can configure Postgres to return only numeric error
codes so that pg_errormessage would return those numerical values that I
can easily test on.

Is this possible? Is this how everyone does it? Am I missing something -
Like a page from the manual? :)

Thanks once again.