When we try to login to admin it shows fatal error. So is there any solution for this problem?
Access to undeclared static property: Mage::$_isDeveloperMode in /home/mominaslib/public_html/app/Mage.php on line 881
I'd give https://magento.stackexchange.com/a/79637/41117 a read.
If you've made any changes to
Mage.php then I suggest replacing it to the default version from your Magento 1 install and trying again.