Codice PHP:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>

<head>
    <meta http-equiv="content-type" content="text/html; charset=iso-8859-1">


    <title>Untitled 1</title>
    <script src="http://code.jquery.com/jquery-latest.js"></script>

    <script type="text/javascript">
    $(document).ready(function(){
    $("button#addRemove").click(function(){
        $("#title").toggle("slow");    
});
    
});
</script>
</head>

<body>

<form name="subm_m" method="post" action="">
<select name="sel_friend" > </select>


<button id="addRemove" />Aggiungi / rimuovi</button>

<input type="text" name="titolo" value="" id="title" />


<textarea name="mess" cols="40" rows="10" ></textarea>

<input type="submit" value="Invia!" name="rapid_mess" /> 

</body>
</html> 
E' troppo facile con jquery!