Your opinion on security issue: file extension

I would like opinions on a security question. A co-worker suggested we rename our application files to some extension other than .php (for example, .htm). The reasoning being that the .php extension tells a cracker that we are using PHP, and not ASP, or ColdFusion, etc. The cracker can focus immediately on vulnerabilities of PHP. So, is there something to be gained by masking our server setup by changing our filename extension? TIA Kirk