PHP-functie aanroepen met HTML-knop

Probeer dit
 <?php  if(isset($_POST['submit'])){
    insert(); 
    }
    ?>

<!DOCTYPE HTML>
  <head>
  <title></title>

  </head>

  <body>
  <div id="main">

  <div id="login">
  <h2>Create User Account</h2>

  <form action="" method="post">
  <br><br><br>
  <label>UserName :</label>
  <br><br>
  <input id="name" name="usernameu" placeholder="username" type="text">
  <br><br><br>
  <label>Password :</label>
  <br><br>
  <input id="password" name="passwordu" placeholder="**********" type="password">
  <br><br><br>
  <input name="submit" type="submit" value=" Create user" >
  </div>
  </div>

  </body>
  </html>