Although the requirements for Joomla 1.5.x says that it is sufficient to have PHP version 4.3.10 or higher, it appears that version 1.5.7 uses stripos() is a PHP 5 feature. So if you webserver is not running PHP5, you may run into this error ...
"Call to undefined function: stripos() in libraries/joomla/environment/uri.php on line 675"
This can occur during user scenarios as user is trying to log in (as described in this forum thread) and when posting an article (as described here).
The solution is to either upgrade the server to PHP 5 or add a bit of code to the Joomla core file /libraries/joomla/utilities/compat/php50x.php as described in the above mentioned forum threads. The code essentially detects if the stripos function exists or not. If not, it will use a different algorithm that employs strpos (which is a PHP4 function) instead of stripos.
If you find that Joomla 1.5.7 with SEF turned on is prefixing the category links with the id in the URL when you don't want it, consider reading this bug description. This affects the file /components/com_content/helpers/route.php
As with many other system, Joomla also has a template override system. It is designed so that you can alter the Joomla core functionality without altering the core code. As mentioned in Joomla community blog, nearly anything can be overridden.
