[horde] Horde 2.2.5 / PHP 5 compatibility

[garnier at embratel.com.br]

Hello all. I have Horde 2.2.5 (I know, VERY OLD) installed with IMP 3.2.4
and PHP 4.4.3. It´s been working fine for years. Now I´ve tried to upgrade
PHP to 5.1.6, but I get the following message when trying to access IMP
login page or Horde test.php page:

Fatal error: Using $this when not in object context

After some Google searches, I guess it´s a PHP 5 backward incompatibility. Can someone say which versions of PHP are compatible with this Horde
version (and vice-versa)?

Thanks