Properly Formatting URL To Locate PHP File

I've included a js file in the header of the site which utilizes a function in a php file. Just the standard <script language="javascript" src="[URL]"></script>
In the .js file I'm setting the location to a php file as a variable...
var url="/file-here.php";
url=document.location.host+url;
Obviously the document.location.host isn't working for me but here's what's going on... At first I just had, var ="[URL]"; in the js file. So if the user is on [URL] the script will work, however if the user is on domain.com/some-directory/another the script won't work, because I've included www. in the variable within the js file.

However, if I set it to var ="[URL]";, it will work on domain.com/some-directory/ but not on [URL]. So basically I need to return the host url somehow, depending on if the user is on the page with www. or without www. so it will always work. With php, I could just use $server['HTTP_HOST']/file-here.php and it will always return www.domain.com or domain.com depending on the URL the user is on, but obviously php won't work in a js file so I'm looking for the correct string/command to get this. How to return just the www.domain.com or domain.com portion of the current URL with javascript? So no matter where they are on the site, with or without www. the script can still function by having the correct URL to the PHP file at all times?