<form style="text-align:center;margin-top:60px" action="" id="deco">
<div>
<span style="font-size:20px">
Bienvenue <?php echo $_SESSION['nom'];?>
</span>
<p>
<input type="button" class="bouton" value="se déconnecter" onclick="location.href='deco.php'" />
</p>
</div>
</form>