Check whether request is coming from frontend or backend.
if(Mage::app()->getStore()->isAdmin() && Mage::getDesign()->getArea() == 'adminhtml'){
//request is from backend/admin
} else {
//request is NOT from backend/admin
}
Changing the Admin URL: Open the /app/etc/local.xml file, locate the <frontName> tag, and change the ‘admin’ part it to something a lot more random.
<frontName><![CDATA[this-is-the-new-url]]></frontName>
Changing the admin theme in local xml
<config>
<stores>
<admin>
<design>
<theme>
<default>yourtheme</default>
</theme>
</design>
</admin>
</stores>
</config>