Before close the connection remember to free your query results too...
$dbh = ibase_connect($host, $username, $password);
$stmt = 'SELECT * FROM tblname';
$sth = ibase_query($dbh, $stmt);
while ($row = ibase_fetch_object($sth)) {
.............
}
ibase_free_result($sth); // <-------
ibase_close($dbh);