Include a file in jQuery (Avgrund modal plugin)

Question

I'm learning "Avgrund Modal plugin" and i'm wondering how i include the contents of a HTML/PHP file with javascript.

When i create the modal window i use this code:

<script>
$(function() {
    $('#show').avgrund({
        height: 200,
        holderClass: 'custom',
        showClose: true,
        showCloseText: 'Stäng',
        onBlurContainer: '.container',
        template: "Helloooo"
    });
});
</script>

The value in template is the content of the modal window. This is it's description in doc

template: 'String content..' // or function (elem) { }, or $('.content')

How do i include a full php/html file to run when i open the window?

Thanks in advance.