You need to set the correct charset set on your ibase session. If not set php follows the default. Since you're already setting the charset you may have data in the database that is not valid as ISO-8859-1.
When connecting to your database, set the charset as the 4th parameter in ibase_connect. You either have to set the "correct" charset or in some cases you could use charset 'NONE' if you use 'NONE' you will not have any character conversions and use more or less raw data.
Try to connect with:
if (!($dbh=ibase_connect($host, $username, $password, 'NONE', 0, 1)))
die('Could not connect: ' . ibase_errmsg());