If there is one thing that bothers me about Dreamweaver’s default settings (other than its annoying habit of rewriting valid code as camelCase), it is that double-clicking .php and .asp files launches Dreamweaver in Design view. Who on earth thinks that people generally want to open these kinds of files in anything other than code view?

Well, thankfully, this ridiculous behavior can be changed with a quick registry tweak. Just add an extension to the “Open As Text” value to cause Dreamweaver to always open that file type in code view. The .reg file below assumes you’re running Dreamweaver CS5.

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Adobe\Dreamweaver CS5\Helper Applications Preferences]
"Open As Text"=".js .asa .css .cs .config .inc .txt .as .asc .asr .vb .htaccess .htpasswd .php .asp .html"

It would be nice if Dreamweaver were smart enough to realize that, if your preferred layout is Coder, any file should be opened in Code view.

The link below explains how to do the same thing via Edit | Preferences.

Source: Open files in Code view by default