I want to call a PHP file using ajax where in that PHP i will place order by the ajax call. But it throws error while i am using app/Mage.php from that file
You should initialize the Magento Framework first:
/* Store or website code */
$mageRunCode = isset($_SERVER['MAGE_RUN_CODE']) ? $_SERVER['MAGE_RUN_CODE'] : '';
/* Run store or run website */
$mageRunType = isset($_SERVER['MAGE_RUN_TYPE']) ? $_SERVER['MAGE_RUN_TYPE'] : 'store';
Mage::init($mageRunCode, $mageRunType, array());