Joomla 1.5 won't work on PHP 5.4

From Joomla! Documentation

Joomla 1.5 is NOT compatible with PHP 5.4.

You must update your site to use Joomla 2.5+ if you find your host upgrades your PHP version.

There is a tutorial here on how to migrate your Joomla version from 1.5 to 3.4+