PHP - Call A Php Function Using Forms?

<?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

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;
      }