Appending values non-destructively

Using PostgreSQL 7.2 with PHP 4.1.2, I would like to use the UPDATE function
to add values to a field in a row that already has a value or values, i.e.:
in a field named 'sponsor', the current value is '2588'. I would like the new
value in 'sponsor' to become '2588, 2961', and later , '2588, 2961, 3166' etc.

From my newbie reading, I haven't discovered whether it is possible to append
values to a field without destroying the current value, and further, whether
comma separators (or any sprators for that fact) are allowed or even possible.

If some kind guru could help out with this simple question, I would greatly
appreciate a small example code showing how to do it.

Tia,
Andre

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