Joomla! Help Site - Minimum System Requirements
You must ensure that you have the MySQL, XML, and Zlib functionality enabled within your PHP installation. This is controlled within the php.ini file.
Do not use PHP 4.3.9, PHP 4.4.2 or PHP 5.0.4; these releases have known bugs that will interfere with the installation of Joomla!
There was also an issue with the Zend Optimizer Version 2.5.10 for PHP 4.4.x. You should ask your host to upgrade to a later release as soon as possible where applicable.