$match = "select id from $table where username = '".$_POST['username']."' and password = '".$_POST['password']."';";

<?php
ob_start();

// Connection a la bd
print "<br><br><br><br><br><br><br>";
$link = mysql_connect("nomduserveur", "loginserveur", "passserveur")
or die ("Could not connect to mysql because ".mysql_error());

// Selection de la database

mysql_select_db("test")
or die ("Could not select database because ".mysql_error());
   
$match = "select id from $table where username = '".$_POST['username']."'
and password = '".$_POST['password']."';";

$qry = mysql_query($match)
or die ("Could not match data because ".mysql_error());
$num_rows = mysql_num_rows($qry);

if ($num_rows <= 0) {
echo "Désolé, il n'y a pas de nom d'utilisateur $username associé au mot de passe inscrit.<br>";
echo "<a href=connectionquiz_form.htm>Réessayez de nouveau</a>";
echo "<a href=inscriptionquiz_form.htm>Inscription</a>";
exit;
} else {

setcookie("loggedin", "TRUE", time()+(3600 * 24));
setcookie("mysite_username", "$username");
echo "Vous êtes maintenant connecté. Bienvenue $username !<br>";
echo "Pour poursuivre, cliquez sur la <a href=quiz01.html>section membre</a>";
}

//FIN DE LA VERIFICATION.

ob_end_flush();

mysql_close($dbh);

?>

if ($_SERVER['REQUEST_METHOD'] == 'POST') {
  if (isset($_POST['username'] and isset($_POST['password']) {
    //...
  } else {
    //...
  }
} else {
  //...
}