After I installed a Joomla component which is only running on PHP5 I tried to change PHP version. So I came across that solution by accident.
Normally my ISP provides PHP4 (1&1 in Germany). That's why it didn't work. But the ISP has PHP5 installed and it is automatically used if files have a .php5 extension. To force using PHP5 for all *.php files I put an .htaccess file in the joomla root dir containing
AddType x-mapp-php5 .php
AddHandler x-mapp-php5 .php
AddHandler x-mapp-php5 .php
I feared that this could cause problems with other things, but until now all is working fine (Joomla 1.5.14).