PHP函数+ Jquery + Ajax

I cant make it work. I have a php script on a website (not so good in php, so if something is wrong, please point me). And i want to call it from a js document.

<?php
    function doit($option){
        if ('getit' == $option){
            $value = '318273918739182739179';
            return $value;
        }else{
            return 0;
        }
    }
?>

and i want to call it from a js file. How do i pass an argument to the php script via Ajax?

var getanswer ={
php: function(){
    $.ajax({
           url:'mywebsite.com/php/return.php',
           data: {action, 'doit'},
           type: 'post',
           success: function(output){
           alert(output);
           }
           })
}
}

in javascript:

$.get('yourPHPscript.php?argument='+some_arg,function(data){
 //data contain result from php script
});

in yourPHPscript:

$option = $_GET['argument'];
if ('getit' == $option){
        $value = '318273918739182739179';
        echo $value;
    }else{
        echo "0";
    }

You can use ajax to call php whenever you need it

for example I have used it for calling a function glob in php

here is some of the jquery code

$('#search_query').keyup(function() {
    var search_input = $("#search_query");
    var output = search_input.val();
    if(output.length > 0){
    $('#app_menu').hide();
    $('#note').show(1000);
        $.ajax({
                type: "POST",
                url: "search_node.php",
                data:{item:output}
            }).done(function( msg ) {
                if(msg!='error')
                {
                    $("div#note").html(msg);
                }
                else
                {

                    $("#debug").append("<br />refresh function failed: ");
                    $("#debug").scrollTop("30");
                }
            });//ajax 

        }
        if  (output.length  == 0){
        $('#note').hide();
        $('#app_menu').show(250);

        }
    });

});

There are some extra stuf in this code that you might not need, hope this helps