MySql : Faire un UPDATE et un INSERT.

<?php
include ('header.html');

$num = $_POST['compte'];
$operation = $_POST['operation'];
$date = $_POST['date'];
$montant = $_POST['montant'];

if ($num != "" && $operation != "" && $date != "" && $montant != "")
{
mysql_connect("localhost","root","");
mysql_select_db("comptes");

mysql_query("INSERT INTO operations VALUES('', '$date', '$operation', '$montant', '$num')"); 

mysql_query("UPDATE comptes SET solde = solde+'$montant' WHERE idcompte = '$num'");

mysql_close();

header ('location: index.php');

}
?>
<div class="fond">

<link rel="stylesheet" type="text/css" href="styles.css" />

<div class="fond">

<br><br><br>
<p style="text-align: center; font-weight: bolder;">Enregistrer une opération</p>
<form action="gestion.php" method="post">
<p style="text-align: center;">
<label for="compte"><small><u>Sélectionner un compte :</u> </small></label><br>
<select name="compte" id="compte"><option></option>
<?php
mysql_connect("localhost","root","");
mysql_select_db("comptes");

$comptes = mysql_query("SELECT * FROM comptes ORDER BY idcompte");

while ($donnee = mysql_fetch_array($comptes))
{
echo '<option>'.$donnee['compte'].'</option>';
}
mysql_close();
?>
</select></p>


<table style="width: 500px; margin-right: auto; margin-left: auto;" cellspacing="0">
<tr class="titres">
<td class="compte" style="padding-left: -20px;">Opération</td>
<td class="solde">Date <small>(AAAA/MM/JJ)</small></td>
<td class="supprimer">Montant</td>
<td class="supprimer">Valider</td>
</tr>
<tr class="compte1">
<td><input type="text" size="50" width="50" name="operation" /></td>
<td><input type="text" name="date" /></td>
<td><input type="text" name="montant" /></td>
<td align="center"><input type="submit" class="boutons" value="v" style="color: green;" /></td>
</tr>
<tr>
<td class="footer" colspan="4"><a href="index.php">Opération dans un autre compte</a></td>
</tr>
</table>
</form>
</div>

<?php

include('footer.html');
?>

<?php



?>
</div>
<?php
include ('footer.html');
?>

mysql_query("INSERT INTO operations VALUES('', '$date', '$operation', '$montant', '$num')");