Getting JSON from PHP script

I a working on an iOS app that needs to get data from a remote server.

This is the Swift code that is getting the JSON array from the PHP script:

func doSearch(_ searchWord: String)
    {
        // Dismiss the keyboard


        // Create URL
        let myUrl = URL(string: "...getAsesores.php")

        // Create HTTP Request
        let request = NSMutableURLRequest(url:myUrl!);
        request.httpMethod = "POST";

        if let x = UserDefaults.standard.object(forKey:"miCodigoAgencia") as? String
        {
            print (x)
        }


        let postString = "searchWord=\(UserDefaults.standard.object(forKey:"miCodigoAgencia"))";
        print (postString)
        request.httpBody = postString.data(using: String.Encoding.utf8);

        // Execute HTTP Request
        URLSession.shared.dataTask(with: request as URLRequest,
                                   completionHandler: { (data, response,error) -> Void in

            // Run new async task to be able to interact with UI
            DispatchQueue.main.async {

                // Check if error took place
                if error != nil
                {
                    // display an alert message
                    self.displayAlertMessage(error!.localizedDescription)
                    return
                }


                do {

                    // Convert data returned from server to NSDictionary
                    let json = try JSONSerialization.jsonObject(with: data!, options: .mutableContainers) as? NSDictionary



                    // Cleare old search data and reload table
                    self.searchResults.removeAll(keepingCapacity: false)


                    // If friends array is not empty, populate searchResults array
                    if let parseJSON = json {

                        if let friends  = parseJSON["friends"] as? [AnyObject]
                        {
                            let primerName = ["Nombre": "Selecciona un asesor" ,"Apellidos": " ", "Id": " ", "Tel": " ", "Email": " " ]
                             self.searchResults.append(primerName)

                            for friendObj in friends
                            {
                                let fullName = ["Nombre": friendObj["nombre"] as! String, "Apellidos": friendObj["apellidos"] as! String, "Id": friendObj["id"] as! String, "Tel": friendObj["tel"] as! String, "Email": friendObj["email"] as! String]

                                self.searchResults.append(fullName)

                            }


                            self.pickerAsesores.reloadAllComponents()

                        } else if(parseJSON["message"] != nil)
                        {
                            // if no friends returned, display message returned from server side
                            let errorMessage = parseJSON["message"] as? String

                            if(errorMessage != nil)
                            {
                                // display an alert message
                                self.displayAlertMessage(errorMessage!)
                            }
                        }
                    }

                } catch {
                    print(error);
                }

            } // End of dispatch_async


        }) // End of data task


        .resume()

    } // end of doSearch() function

Then there is the PHP part on two files:

FILE 1 getAsesores.php:

<?php
require("../db/MySQLDAO.php");


$config = parse_ini_file('../../../SwiftPhp.ini');
$dbhost = trim($config["dbhost"]);
$dbuser = trim($config["dbuser"]);
$dbpassword = trim($config["dbpassword"]);
$dbname = trim($config["dbname"]);


$dao = new MySQLDAO($dbhost, $dbuser, $dbpassword, $dbname);
$dao->openConnection();

$searchWord = null;

if(!empty($_REQUEST["searchWord"]))
{
   $searchWord = htmlentities($_REQUEST["searchWord"]);
}

$friends = $dao->buscarAsesores($searchWord);


if(!empty($friends))
{
    $returnValue["friends"] = $friends;
} else {
    $returnValue["message"] = "Could not find records";
}

$dao->closeConnection();

echo json_encode($returnValue);

?>

And file 2: MySQLDAO.php:

public function buscarAsesores($searchWord)
{
    $returnValue = array();

    $sql = "select * from members LEFT JOIN tb_agencias ON members.agencia_usuario = tb_agencias.id_agencia where 1";

    if(!empty($searchWord))
    {
        $sql .= " and ( codigo_agencia like ?  )";
          $sql .= " ORDER BY nombre";

    }

    $statement = $this->conn->prepare($sql);

    if (!$statement)
        throw new Exception($statement->error);

    if(!empty($searchWord))
    {
      $searchWord = '%' ;
      $statement->bind_param("s",  $searchWord );
    }

    $statement->execute();

    $result = $statement->get_result();

     while ($myrow = $result->fetch_assoc()) 
     {
       $returnValue[] = $myrow;
     }

    return $returnValue;
} 

If I call the PHP directly from the browser, the result is not correct, the query is not filtering the records as I want, and the iOS app shows all records.

If I remove the line:

  $searchWord = '%' ;

as commented in my previous question here at SO Weird filter result in PHP function

at MySQLDAO.php, then the query filters the records as I want, but the app shows the message: Could not find records

Any help is welcome.

EDITED CODE UPDATED TO SWIFT 3 FROM THE CORRECT ANSWER BY @Chris:

 if let x = UserDefaults.standard.object(forKey:"miCodigoAgencia") as? String {
            print (x)
            var postString = "searchWord=\(x)".addingPercentEncoding(withAllowedCharacters:NSCharacterSet.urlQueryAllowed);
            print (postString)
            request.httpBody = postString?.data(using: String.Encoding.utf8)
            request.setValue("\(request.httpBody?.count)", forHTTPHeaderField:"Content-Length")
        }