Codice PHP:
<?
$datafile = "data.txt"; 
$nameErr = FALSE;
if(isset($_POST['submit'])){ 

$name = trim($_POST['name']); 
$comments = trim($_POST['comments']); 

if($name=="" || $comments==""){
$nameErr = TRUE;
}else{
$new_content = "$name::$comments\n"; 

$fp = fopen($datafile, "a"); 
$fw = fwrite($fp, $new_content); 
$fc = fclose($fp); 
header("Location: ".$_SERVER['PHP_SELF']."");//aggiunta 
}
} 

?> 

<html> 
<body bgcolor="#FFFFFF" text="#000000"> 
[b]GUESTBOOK [/b] 
<form name="form1" method="post" action="<?php echo $_SERVER['PHP_SELF']; ?> "> 
<table width="30%" border="0" cellspacing="0" cellpadding="0" height="50"> 
<tr> 
<td width="31%">Name:</td> 
<td width="69%"><input type="text" name="name">
<?
if($nameErr){
echo "<font color='red' size='2'> Inserisci un nome</font>";
}
?>
</td> 
</tr> 
<tr> 
<td width="31%">Comments:</td> 
<td width="69%"><textarea name="comments" rows="3"><? if($nameErr){
echo $comments;
}
?></textarea></td> 
</tr> 
</table> 
<input type="submit" name="submit" value="Submit Comment"> 
<input type="reset" name="Reset" value="Reset"> 

 
</form> 
<hr> 

<?php 

if (file_exists($datafile)) { 
$read_data = file($datafile); 
$num_comments = count($read_data); 

for($c=0;$c<$num_comments;$c++){//prima era for($c=0;$c<$num_comments-1;$c++){ 

$content = explode("::", $read_data[$c]); 

echo "[b]Name:[/b] $content[0]"; 
echo "
"; 
echo "[b]Comment:[/b] $content[1]"; 
echo "

"; 
} 
} 
?> 
<hr> 
</body> 
</html>
Puoi fare così. Ho messo che controlla anche il commento, ma l'echo dell'errore mettilo tu va :P