Champ obligatoire php

Bonjour,

Voilà je débute en phph et je fais un petit forum, juste un mini-ptojet, mais j'ai un souci.. J'aimerais par exemple que le champ Username soit obligatoire
J'ai trouvé une solution en php mais je comprends pas pourquoi ça ne marche pas :

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<?php
$username = $_POST['username'];
$password = $_POST['password'];
if(empty($username))
{
print("<center>Le '<b>Name</b>' est vide !</center>");
exit();
}
?>

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Et aussi.. Question : où dois je positionner dans le code html ?

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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
<title>Login Page</title>
<link href="design.css" rel="stylesheet" type="text/css">
<link rel="shortcut icon" href="blabla.png" type="image/x-icon"/>
<link rel="icon" href="blabla.png" type="image/x-icon"/>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.1/jquery.min.js"</script>

</head>
<body>
<div id="header">

<h1>SupForum</h1>

</div> <div id="menu">
<ul>
<li><a href="index.php">Categories</a></li>
<li><a href="login.php">Topics</a></li>
<li><a href="login.php" class="right">Login</a></li>
<li><a href="index.php" class="right">Register</a></li>
</ul>

</div> <div id="content">

<h1>Register Page</h1>

<form method="post" action="page.php" enctype="multipart/form-data">

<fieldset>
<legend> Identification Informations : </legend>
<div>
<label for="username">Username: </label>
<input id="username" type="text" name="username" value="" />
<span style="color:red" ></span>
<!--<span class="info"></span>-->

</div>
<div>
<label for="password">Password:</label>
<input id="password" type="password" name="password" value="" />
</div>
<div>
<label for="password">Password Confirmation:</label>
<input id="passwordconf" type="password" name="passwordconf" value="" />
</div>
</fieldset>
<fieldset>
<legend> Personal Informations : </legend>
<div>
<label for="firstname">Firstname: </label>
<input id="firstname" type="text" name="firstname" value="" />
</div>
<div>
<label for="lastname">Lastname:</label>
<input id="lastname" type="text" name="lastname" value="" />
</div>
<label for="email">Email: </label>
<input id="email" type="text" name="email" value="" />
</fieldset>
<div>
<label for="avatar"> Avatar:</label>
<input id="avatar" type="file" name="nom" />
<input type="hidden" name="MAX_FILE_SIZE" value="1048576" />
</div>
<div>
<input type="submit" value="Submit" />

</div>
</div>

</form>

</div>
</body>
</html>

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Merci ! =D