これを試してください
<?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>