What i am doing wrong here ?

<?php
require 'db.php';

   function submit($data){
        $column = implode(",", array_keys($data));
        $values = "'".implode("','", $data)."'";

        try{
          global $db;
          $insert = $db->prepare("insert into user_data($column) values(?)");
          $insert -> bindParam(1,$values); 
        }catch(Exception $e){
            $e->getMessage();
        }
        $insert->execute();

   }

  if(isset($_POST) && !empty($_POST)){

     $userName = filter_input(INPUT_POST, 'userName' ,FILTER_SANITIZE_STRING);
     $email = filter_input(INPUT_POST, 'email' ,FILTER_SANITIZE_STRING);
     $password = filter_input(INPUT_POST, 'password' ,FILTER_SANITIZE_STRING);
     $confirm = filter_input(INPUT_POST, 'confirm' ,FILTER_SANITIZE_STRING);



     if($userName == "" || $email == "" || $password == "" || $confirm == ""){
        echo '<span class="error">Empty Fields</span>';
     }elseif(strlen($userName) < 4){
        echo '<span class="error">Username must be greater than 4 characters</span>';
     }elseif(!filter_var($email,FILTER_VALIDATE_EMAIL)){
        echo '<span class="error">Invalid email address</span>'; 
     }elseif($password < 8){
        echo '<span class="error">Password must be more than 8 characters</span>';
     }elseif($password !== $confirm){
        echo '<span class="error">Password not match</span>';
     }else{
        $cost = [md5($userName)."12"];
        $password = password_hash($password,PASSWORD_DEFAULT,$cost);
        $data = [
            'userName' => $userName,
            'email'    => $email,
            'password' => $password
        ];

        submit($data);

     }


  }