how to resolve php soap function?

Question

i am trying to create a soap web service.i have 3 files a client,a server and the function. The function tries to get the login credentials from the database.The problem seem to be in the function file.When i tried to exceute the function file by removing the function statement, everything works but as soon as i put back the function statement and tries to test the function from another function,it return nothing.

here is my function file:

 <?php
    start();
    function login($username,$password)
   {

  include 'includes/db.php';
  $data = null;
  $sqlstring = "SELECT * FROM accountdetails WHERE username='$username' AND password='$password';";
  $result=mysqli_query($con,$sqlstring);

  if( mysqli_num_rows($result) > 0){

     while($rows = mysqli_fetch_assoc($result)){
    $userid = $rows["userid"];
    $firstname = $rows["firstname"];
    $lastname = $rows["lastname"];
    $username = $rows["username"];
    $email =  $rows["email"];
    $data = $userid.",".$firstname.",".$lastname.",".$username.",".$email;
   //make it into a csv string
    }
   }

  return $data;

 } 

function start(){
 $data1 = login("neem","tas");
  echo $data1;///return nothing
 }
?>

here is my soap server file

 <?php

  require 'function-soap.php';
  require 'lib/nusoap.php';
  $server = new nusoap_server();
  $server->configureWSDL("maleehahweb","uri:maleehahweb");
  $server->register(
   "login",
   array("username"=>"xsd:string","password"=>"xsd:string"),
    array("return"=>"xsd:string"));

    $HTTP_RAW_POST_DATA = isset($HTTP_RAW_POST_DATA) ? $HTTP_RAW_POST_DATA :"";
   $server->service(file_get_contents("php://input"));

   ?>

here is my client file

   <?php

       require 'lib/nusoap.php';
      $client = new nusoap_client("http://localhost/maleehahweb/server_soap.php?wsdl",true);

       $error = $client->getError();

       if($error){
           echo $error;


       $username = "neem88";
       $password = "tas";
       $userid1 = null;
       $firstname1 = null;
       $lastname1 = null;
       $username1 = null;
       $email1 = null;
       $parameter = array("username"=>"$username","password"=>"$password");

       $result = $client->call("login",$parameter);
       var_dump($result);

       /*list($userid1, $firstname1, $lastname1,$username1,$email1) = explode(',', $result)*/

    /*$_SESSION["userid"] = $userid1;
    $_SESSION["firstname"] = $firstname1;
    $_SESSION["lastname"] = $lastname1;
    $_SESSION["username"] = $username1;
    $_SESSION["email"] =  $email1;*/


        echo $userid1;//does not output anything    
       if($client->fault){
           print_r($price);

                  }
       ?>