In the folder with the JavaScript, the .htaccess file should include:
In that above example, any file that includes the string "api" will be processed as PHP. Feel free to alter that RegEx.
That will ensure browsers reading the file will interpret it as JavaScript. Now you can use <php ... ?> tags in the JavaScript file to do whatever PHP stuff you need to do.

Then in the JavaScript file itself, set the ContentType back to JavaScript: