If you want to force a save-as dialog box to open for a file type that your web server is configured to output as a readable document (and you don't want to change that configuration), you need to create a file that modifies the headers so that the browser creates such a dialog.

See the very first example in the php documentation for readfile.

You would then create a PHP file that could be accessed through a normal a href link, which would send the appropriate headers and output myFile.txt.