PHP文件如何通过此回调参数了解和处理函数？

I am finding way to get data JSON from PHP file without knowing the function PHP file is processing.

<script>
function clickButton(){
    var s = document.createElement("script");
    s.src = "jsonp_demo_db.php?callback=myDisplayFunction";
    document.body.appendChild(s);
}
</script>

send the function name with get request and check if is Get value exist and use call_user_func($functionName) and if there is parameter with function you can use call_user_func_array($functionName , $pramaters)

`function test()
{
  echo "string";
}

if ($_GET['func']) {
  $func = $_GET['func'];
  call_user_func($func);
}`

you can also use function_exists to enshure if the function exist and this function return False or True this code Example :

if(function_exists('my_function'))
{
throw new Exception("'my_function' is already defined!");
}

function my_function()
{
  // Do the work here
}