How a php file can be called from a web app? [duplicate]

I have a simple web app which runs a server using a certain port, Is it possible to use an ajax function from inside its index.html to call a php file, as the following:

    {
    $.ajax({
        url:"fetch.php",
        method:"post",
        data:{query:query},
        success:function(data)
        {
            $('#result').html(data);
        }
    });
}

A B4J (Java) app runs the server with a certain port, which can be launched on a web browser. The php file is called from inside the home page (html) of the Java app server. On a usual server (like localhost on a PC) the php file is executed without any problem, but here it is not.

source https://stackoverflow.com/questions/70022968/how-a-php-file-can-be-called-from-a-web-app