php mysql warning mysql_num_rows

Question

I am adding some php code for a website that is hosted for a couple of years now. I check everything on localhost and is just fine. But when I uploaded the new code to the host I got a warning:

Warning: mysql_num_rows() expects parameter 1 to be resource, boolean given in /home/blah/blah

I have a class for mysql connection:

<?php

class ConnDB {

const SERVER = "localhost";
const USER = "admin";
const PASSWORD = "admin";
const NAMEDB = "mydb";

private $myConn;
private $status;

function __construct() {
    $this->myConn = mysql_connect(self::SERVER, self::USER, self::PASSWORD);

    if(!$this->myConn) {
        $this->status = false;
    } else {
        if(!mysql_select_db(self::NAMEDB)) {
            $this->status = false;
        } else {
            $this->status = true;
        }
    }
}

public function getMyConn() {
    return $this->myConn;
}

public function getStatus() {
    return $this->status;
}
}

Then I have my class that is using the connection:

<?php

require_once 'clase.conn.php';

class User extends ConnDB {

public function usersList() {
    $query = "SELECT USER_ID, NAME, MAIL FROM USER";
    $result = mysql_query($query, $this->getMyConn());
    if(mysql_num_rows($result) == 0) {
        return 0;
    } else {
        while($row = mysql_fetch_assoc($result)) {
            $myarray[] = $row;
        }
        return $myarray;
    }
}

I added a validation to check if result was true or false, thinking that was because the DB was empty. But when i added some rows through phpmyAdmin, i still got the same warning.

What could it be?