Php/mysql Notice: Undefined index: submit

Bonjour,
Il y'a mon code sign.php pour inserer les donnees dans la base qui me renvoi cette erreure que je comprends pas:
Notice: Undefined index: submit in C:\Program Files\EasyPHP 2.0b1\www\training\application\sign.php on line 6

voici le code
sign.php

<?php
include "connection.php";
include ("usefulfunc.php");
$page_title = "Sign My Guest Book!!";
include "start_pag.php";
$submit=$_POST["submit"];
if ($submit == "Sign!"){
$errmsg = create_entry($name,$location,$email,$url,$comments);
if (empty($errmsg))
{
include "end_page.php";
exit;
}
}
?>

<form method=POST>
<table>
<?php

print_input_fields("name","location","email","url");
?>
<tr>
<td valign=top align=rigth><b>Comments:</b></td>
<td valign=top align=left><textarea name=comments cols=40 rows=4></textarea></td>
</tr>

</table>
<input type=submit name=submit value="Sign!">
<input type=reset name=reset value="Start Over">

</form>

Et voici la function create_entry pour analyser et inserer les donnees dans la base:

function create_entry($name,$location,$email,$url,$comments){
$errmsg = "";
if(empty($name)){
$errmsg .= "<li>you have to put in a name!\n";
}
if (empty($email)){
$errmsg .="<li>insert an email please\n";
}
else {
$query = "select * from guestbook where email = '$email'";
$result = safe_query($query);
if(mysql_num_rows($result) > 0) {
$errmsg .="<li>this email has already sign this guestbook.\n";
}
}
if (empty($errmsg)){
$query = "insert into guestbook"
."(name,location,email,url,comments) values "
."('$name','$location','$email','$url','$comments')";
$result = mysql_query($query);

?>
<h2>Thanks!!</h2>
<h2><a href="view.php">View Arly's Guest Book!!!</a></h2>
<?php

}
else{
include("sign.php");
}

}
?>


Merci d'avance pour votre reponse.