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First of all, check if Oracle PHP extension is actually installed. If it's installed, is the extension enabled in php.ini? Run phpinfo() and search "Oracle". If you don't see it, then Oracle extension is not installed or enabled.
Configuring PHP with Oracle: http://www.php.net/manual/en/oci8.installation.php You can also use Oracle TNS.(Transparent Network Substrate). In conf.php set PHPGRID_DB_HOSTNAME value to a TNS name instead as documented on http://phpgrid.com/documentation/installation/