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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 upgrade your Joomla version from 1.5 to 2.5+