PHP - Call Function W/ Onclick W/ Php_self

<?PHP
require_once("./include/membersite_config.php");
if(!$fgmembersite->CheckLogin())
{
    $fgmembersite->RedirectToURL("login.php");
    exit;
}
if(isset($_POST['submitted']))
{
$fgmembersite->PesquisarPorDatas();
}

?>
               
			<div id='fg_membersite_content'>
			<div class="CSSTableGenerator" >
				//I want the result here
			</div>
			<br>
			

I've made this code for a very simple registration form practice:

  //assign form variables
  $firstname = $_POST['firstname'];
  $lastname = $_POST['lastname'];
  $username = $_POST['username'];

  function clean_names($dirty_name) {
      strip_tags($dirty_name);
      str_replace(' ', '', $dirty_name);
      ucfirst(strtolower($dirty_name));

      //return $dirty_name;
    }

  $names = array($firstname, $lastname, $username);

  if(isset($_POST['register_button'])) {

    //  WHY IS THIS FUNC CALL NOT WORKING??
    foreach($names as $name) {
      clean_names($name);

    }

    echo "First Name: " . $firstname . "<br>";
    echo "Last Name: " . $lastname . "<br>";
    echo "Username: " . $username . "<br>";

  }

The values are returned but they haven't been put through the clean_names function.
I'm really new with PHP. Can someone tell me why this isn't working? Thank you

Call to a member function bind_param() on a non-object in /models/class.newuser.php on line 131

$stmt->bind_param("ssssi", $this->username, $secure_pass, $this->clean_email, $this->activation_token, $this->user_active);
$stmt->execute();
$inserted_id = $mysqli->insert_id;$stmt->close();
//Insert default permission into matches table
$stmt = $mysqli->prepare("INSERT INTO ".$db_table_prefix."user_permission_matches (
user_id,
permission_id
)
VALUES (
?,
'1'
)") ;


$stmt->bind_param("s", $inserted_id);
$stmt->execute();
$stmt->close();

<?php
/*
.
.
*/


class User 
{
	public $user_active = 0;
	private $clean_email;
	public $status = false;
	private $clean_password;
	private $username;
	public $sql_failure = false;
	public $mail_failure = false;
	public $email_taken = false;
	public $username_taken = false;
	public $activation_token = 0;
	public $success = NULL;
	
	function __construct($user,$pass,$email)
	{

		//Sanitize
		$this->clean_email = sanitize($email);
		$this->clean_password = trim($pass);
		$this->username = sanitize($user);
		
		if(usernameExists($this->username))
		{
			$this->username_taken = true;
		}
		else if(emailExists($this->clean_email))
		{
			$this->email_taken = true;
		}
		else
		{
			//No problems have been found.
			$this->status = true;
		}
	}
	
	public function userCakeAddUser()
	{
		global $mysqli,$emailActivation,$websiteUrl,$db_table_prefix;
		
		//Prevent this function being called if there were construction errors
		if($this->status)
		{
			//Construct a secure hash for the plain text password
			$secure_pass = generateHash($this->clean_password);
			
			//Construct a unique activation token
			$this->activation_token = generateActivationToken();
			
			//Do we need to send out an activation email?
			if($emailActivation == "true")
			{
				//User must activate their account first
				$this->user_active = 0;
				
				$mail = new userCakeMail();
				
				//Build the activation message
				$activation_message = lang("ACCOUNT_ACTIVATION_MESSAGE",array($websiteUrl,$this->activation_token));
				
				//Define more if you want to build larger structures
				$hooks = array(
					"searchStrs" => array("#ACTIVATION-MESSAGE","#ACTIVATION-KEY","#USERNAME#"),
					"subjectStrs" => array($activation_message,$this->activation_token,$this->username)
					);
				
				/* Build the template - Optional, you can just use the sendMail function 
				Instead to pass a message. */
				
				if(!$mail->newTemplateMsg("new-registration.txt",$hooks))
				{
					$this->mail_failure = true;
				}
				else
				{
					//Send the mail. Specify users email here and subject. 
					//SendMail can have a third parementer for message if you do not wish to build a template.
					
					if(!$mail->sendMail($this->clean_email,"New User"))
					{
						$this->mail_failure = true;
					}
				}
				$this->success = lang("ACCOUNT_REGISTRATION_COMPLETE_TYPE2");
			}
			else
			{
				//Instant account activation
				$this->user_active = 1;
				$this->success = lang("ACCOUNT_REGISTRATION_COMPLETE_TYPE1");
			}	
			
			
			if(!$this->mail_failure)
			{
				//Insert the user into the database providing no errors have been found.
				$stmt = $mysqli->prepare("INSERT INTO ".$db_table_prefix."users (
					user_name,
					password,
					email,
					activation_token,
					last_activation_request,
					lost_password_request, 
					active,
					title,
					sign_up_stamp,
					last_sign_in_stamp
					)
					VALUES (
					?,
					?,
					?,
					?,
					?,
					'".time()."',
					'0',
					?,
					'New Member',
					'".time()."',
					'0'
					)");
				
				$stmt->bind_param("ssssi", $this->username, $secure_pass, $this->clean_email, $this->activation_token, $this->user_active);
				$stmt->execute();
				$inserted_id = $mysqli->insert_id;
				$stmt->close();
				
				//Insert default permission into matches table
				$stmt = $mysqli->prepare("INSERT INTO ".$db_table_prefix."user_permission_matches  (
					user_id,
					permission_id
					)
					VALUES (
					?,
					'1'
					)") ;
					
					
				$stmt->bind_param("s", $inserted_id);
				$stmt->execute();
				$stmt->close();
			}
		}
	}
}

?>

class User
    {  private $_db;
       
       public function __construct($user = null)
       {
          $this->_db = DB::getInstance();
           .......
       }
       public function find($user = null)
       { if($user)
         { $field = (is_numeric($user)) ? 'id' : 'username';
         $data = $this->_db->get('users', array($field, '=', $user));
           if($data->count())
           { .........
             return true;
           }
         }
         return false;
       }

public static function getInstance()
      { if(!(self::$_instance)): self::$_instance = new self();
        endif;
        return self::$_instance;
      }