As a manner of good practice, it's best to use an .asp extension for include files. This will keep users who know the include file name from downloading the file. ASP files must be parsed before they are downloaded-thus protecting the include file.
As a side note, some people also prefer to use the word Include in the filename to designate that the file is an include file, however they keep the .asp extension to protect the file.
Source: Sarah Remelt
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