If you want to connect to interbase server runned on custom port (not default port 3050), you can specify it in "database" parameter in this way: 'hostname/port:' (TCP/IP with custom port).
Examples:
$dbh = ibase_connect('example.com/11099:dbalias', $username, $password);
$dbh = ibase_connect('example.com/11099:var/data/myibase.fdb', $username, $password);
$dbh = ibase_connect('example.com/11099:C:\\data\\mywinibase.fdb', $username, $password);