<?php
if (isset($_GET['suppression']))
{
$suppression = $_GET['suppression'];
$reponse30 = mysql_query('DELETE FROM messages WHERE lesujet="'.$suppression.'"');
}
?>

<h1>Messages</h1>

<?php
$reponse25 = mysql_query('SELECT * FROM messages ORDER BY id DESC WHERE aqui="'.$_SESSION['nom'].'"');
if (mysql_num_rows($reponse25) == 0)
{
echo '<center>Vous n\'avez pas de messages</center>';
exit() ;
}
echo '<p><center><table border bordercolor="red" width="500" height="100"><tr width="50%">
                <td align="center" width="10%"><big><b>Expéditeur</b></big></td>
                <td align="center"><big><b>Sujet</b></big></td>
                <td align="center" width="10%"><big><b>Suppression</b></big></td>
                        </tr>';
while ($donnees25 = mysql_fetch_array($reponse25))
{
    echo '<tr>
                <td align="center">'.$donnees25['dequi'].'</td>
                <td align="center"><a href="index1.php?page=messages2&message='.$donnees25['lemessage'].'">'.$donnees25['lesujet'].'</a> </td>
                <td align="center"><a href="index1.php?page=messages&suppression='.$donnees25['lesujet'].'">Supprimer</a></td>
                        </tr>';
}
?>
</table></center>

Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in C:\wamp\www\messages1.php on line 13
Vous n'avez pas de messages

if ( !@mysql_num_rows($reponse25))
{
echo '<center>Vous n\'avez pas de messages</center>';
exit() ;
}

$reponse25 = mysql_query("SELECT * FROM messages ORDER BY id DESC WHERE aqui='".$_SESSION['nom']."'");

<?php
if (isset($_GET['suppression']))
{
$suppression = $_GET['suppression'];
$reponse30 = mysql_query('DELETE FROM messages WHERE lesujet="'.$suppression.'"');
}
?>

<h1>Messages</h1>

<?php
$reponse25 = mysql_query("SELECT * FROM messages ORDER BY id DESC WHERE aqui='".$_SESSION['nom']."'");
if (mysql_num_rows($reponse25) == 0)
{
echo '<center>Vous n\'avez pas de messages</center>';
exit() ;
}
echo '<p><center><table border bordercolor="red" width="500" height="100"><tr width="50%">
                <td align="center" width="10%"><big><b>Expéditeur</b></big></td>
                <td align="center"><big><b>Sujet</b></big></td>
                <td align="center" width="10%"><big><b>Suppression</b></big></td>
                        </tr>';
while ($donnees25 = mysql_fetch_array($reponse25))
{
    echo '<tr>
                <td align="center">'.$donnees25['dequi'].'</td>
                <td align="center"><a href="index1.php?page=messages2&message='.$donnees25['lemessage'].'">'.$donnees25['lesujet'].'</a> </td>
                <td align="center"><a href="index1.php?page=messages&suppression='.$donnees25['lesujet'].'">Supprimer</a></td>
                        </tr>';
}
?>
</table></center>

$reponse25 = mysql_query("SELECT * FROM messages ORDER BY id DESC WHERE aqui='".$_SESSION['nom']."'") or die(mysql_error());