Question

function ajaxCheck(){ 
    var inpUs = $("#user").val(); 
    $.post(
        "./inpCheck.php",
        { user : inpUs }, 
        function(data) { // here is the point
            if (data.length > 0) { 
                $("#spanUser").html(data); 
            } 
        }
    ) 
} 

This checks if a user already exists, and it works, but instead of $.post("./inpCheck.php", I want to refer to a specific part (function) of that file ?
Because, on the same file I want to write code for another inputs.

Answer 1

That’s not possible, what you can do is use switch in php, like

js part

$.post("./inpCheck.php", {user : inpUs, act_name : "some_function"}, function(data){
..

and in inpCheck.php

$action = $_POST['act_name'];
switch($action) {
  case "some_function":
     call_some_function();
  break;
}
//somewhere in same page
function call_some_function() {
  //your code here
}