How do I make a default syntax by filetype in Atom text editor?

This is now in Atom core, you don’t need that file-types package any more.

Where I work, we use .phl for a certain kind of PHP file. To tell Atom about this, first find the language’s Scope Name (e.g. text.html.php ), then edit config.cson like so:

core:
    customFileTypes:
      "text.html.php": [
        "phl"
      ]
    themes: [
// snip

You can find this file at ~/.atom/config.cson, or from the Settings window click the “Open Config Folder” button.

It’s easy to get wrong, as evidenced by the many errors people made on the GitHub issue for the feature.