<a onclick="call_php('php_function_name', ['arg_1', 'arg_2']);">call</a>
<p id="demo"></p>

function call_php(fname, args) {
    var xhttp = new XMLHttpRequest();
    xhttp.onreadystatechange = function () {
        if (this.readyState == 4 && this.status == 200) {
            document.getElementById('demo').innerHTML = this.responseText;
        }
    };
    xhttp.open('POST', 'includes/functions.php', true);
    xhttp.setRequestHeader('Content-type', 'application/json');
    xhttp.send(JSON.stringify({
        'fname': fname,
        'args': args
    }));
}

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    //header('Content-Type: application/json');
    $post = json_decode($_POST/*, true*/);
    // DO SOMETHING TO CALL: fname(...args);
}
function php_function_name(arg_1, arg_2) {
    // A FUNCTION...
}

$postdata = file_get_contents("php://input");

<?php

switch ($_SERVER['REQUEST_METHOD']) {
  case 'POST':
    post_handler();
    return;
  case 'GET':
    get_handler();
    return;
  default:
    echo 'Not implemented';
    return;
}

function post_handler() {
  $raw_data = file_get_contents("php://input");
  $req_body = json_decode($raw_data, TRUE);
  call_user_func_array($req_body['function'], $req_body['args']);
}

function get_handler() {
  echo 'This page left intentionally blank.';
}

function some_function($arg1, $arg2) {
  echo $arg1.' '.$arg2;
}

?>