I put in a pile of code for 3.0.6 to deal with this because our piece
of crap databases crap out all the time.  The way it's intended to
work now is that sybase_pconnection() will not return a known bad
connection.  However, the connection it will return is also not known
to still be good.  If the database is restarted or the machine it's
running on is rebooted it will return a connection that will fail, but
that connection will be marked dead and will not be returned again.
Trying to guess whether the connection is known good is unreliable
because there's always a race where the connection may be good when it
is returned but the database may go down soon afterwards.  You always
need error recovery code in your php script.

I'm closing this because the functionality described above works satisfactorily
for me when the database is restarted or its host is rebooted.

If somebody can suggest a specific way to do something better, please do.