Registration form using javascript and php

[harman30]

In my code below I am using javascript and php for building registration form. There are two files register.php that contains html code for form, javascript library and javascript code. Second file reghand.php handles php code for validation and inserting data. Data is inserted correctly into database after form validation but problem appears when validation fails. It's like no javascript comes into action, when validation fails I get redirected to reghand.php where validation errors are shown {"username":"inuse","password":"missmatch"} these are errors from my reghand.php but I want to use javascript error message that's on register.php and that too should appear on the form page itself, I was never supposed to redirect to reghand.php. Please help.

register.php

    <script type="text/javascript" src="./js/jquery.min.js"></script>
           <script>
            $(document).ready(function(){
      $("form.register").change(function() {
        $.post("register.php", $("form.register").serialize(), function( data ) {
          if( data.flname == "cntbempty" )
            $("p#name_error").slideDown();
          else
            $("p#name_error").hide();
          if( data.username == "inuse" )
            $("p#username_error").slideDown();
          else
            $("p#username_error").hide();
          if( data.password == "missmatch" )
            $("p#password_error").slideDown();
          else
            $("p#password_error").hide();
          if( data.email == "notvalid" )
            $("p#email_error").slideDown();
          else
            $("p#email_error").hide();
        }, "json");
      });
    });
    </script>
       <form action="reghand.php" method="post" class="register">
       <fieldset>
       <legend>Registration</legend>
       <input type="text" name="fname" required="required" maxlength="15" placeholder="First Name"/>        
       <input type="text" name="lname" required="required" maxlength="15" placeholder="Last Name"/>
       <p id="name_error" class="error">First name Last name cannot be empty and can be same</p>
       <input type="text" name="username" required="required" maxlength="15" placeholder="Username" class="username" />        
       <p id="username_error" class="error">That Username is unavailable</p>       
       <input type="email" name="email" required="required" maxlength="35" placeholder="Email"/>        
       <p id="email_error" class="error">That Email is already registered</p>       
       <input type="password" name="password" required="required" maxlength="15" placeholder="Password"/>        
       <input type="password" name="password2" required="required" maxlength="15" placeholder="Confirm Password"/>
       <p id="password_error" class="error">Passwords do not match</p>
       <input type="submit" name="reg" value="Sign Up!">
     </fieldset>
    </form>

reghand.php

    <?php
    if(isset($_POST['reg'])){
     $fn = ucfirst($_POST['fname']);
     $ln = ucfirst($_POST['lname']);
     $un = $_POST['username'];
     $em = $_POST['email'];
     $pswd = $_POST['password'];
     $pswd2 = $_POST['password2'];
     $sql=$db->prepare("SELECT username FROM users WHERE username=:username");
     $sql->execute(array(':username'=>$un));
     $row = $sql->fetch(PDO::FETCH_ASSOC);
     $db_username = $row['username'];
     $usernames = $db_username;
     $data = array();
     if( isset($fn) && isset($ln) ) {
      if( $fn != "" && $ln!="" && $fn == $ln ) {
        $data["flname"] = "cntbempty";
       }
      }
     if( isset($un) ) {
      if ($un == $usernames )  {
        $data["username"] = "inuse";
       }
      }
     if( isset($pswd) && isset($pswd2) ) {
      if( $pswd2 != "" && $pswd != $pswd2 ) {
        $data["password"] = "missmatch";
       }
      }
      if( isset( $em ) ) {
       if( $em != "" && !preg_match( "/^([a-zA-Z0-9])+([a-zA-Z0-9\._-])*@([a-zA-Z0-9_-])+([a-zA-Z0-9\._-]+)+$/", $_POST["email"] ) ) {
         $data["email"] = "notvalid";
       }
      }
     if(!empty($data))
     {
     echo json_encode($data);
     die;
     }
      else{
      $pswd = password_hash($pswd, PASSWORD_DEFAULT);
      $pswd2 = password_hash($pswd2, PASSWORD_DEFAULT);
      $stmt = $db->prepare("INSERT INTO users (username,first_name,last_name,email,password,password2,) VALUES (:username,:first_name,:last_name,:email,:password,:password2,)");  
      $stmt->execute( array(':username'=>$un,':first_name'=>$fn,':last_name'=>$ln,':email'=>$em,':password'=>$pswd,':password2'=>$pswd2));
      }
      if ($stmt->rowCount() == 1) {
        header("Location: login.php");
      } 
      else {
        echo "error";
      }
    }
    ?>