Comment sécuriser mon site

<?php
session_start();

if(isset($_SESSION['pseudo']))
{

?>
<meta charset="utf-8">
<?php
include("afficher.func.php");
 $bdd = new PDO('mysql:host=sql1.olympe.in;dbname=mabdd;charset=utf8', 'monuser', 'monpass');

if (isset($_POST['submit']))
{
     
     
 $pseudo = htmlspecialchars(trim($_SESSION['pseudo']));
 $corps = htmlspecialchars(trim($_POST['corps']));

 
 if(empty($pseudo) || empty($corps))
 {
  echo"Veuillez completer tous lezs champs";

 } else{
 inserer_article($pseudo,$corps);
 }
 
  



}
?>
<form method="post" action="">

<p>Votre article</p>
<textarea name="corps" cols="20" rows="7"></textarea><br><br>

<input type="submit" name="submit" value="Poster" />

</form>

<hr/>

<?php

try
{
// On se connecte à MySQL
 $bdd = new PDO('mysql:host=sql1.olympe.in;dbname=mabdd;charset=utf8', 'monuser', 'monpass');
}
catch(Exception $e)
{
 // En cas d'erreur, on affiche un message et on arrête tout
 die('Erreur : '.$e->getMessage());
}
$req = $bdd->prepare('SELECT * FROM articles ORDER BY date DESC');

$afficher_articles = afficher_articles();

foreach($afficher_articles as $afficher_article)
{

echo "Poster par ".$afficher_article['pseudo']."<br>";
echo $afficher_article['corps']."<br>";
echo "Le ".date('d/m/Y à H:i:s',strtotime($afficher_article['date']))."<br><br>"; 
?>
<a href="inserer.php?id=<?php echo $afficher_article['id_article']?>">Poster un commentaires</a> - <a href="afficher_commentaire.php?id=<?php echo $afficher_article['id_article']?>">Voire commentaires</a>

<hr/>
<?php
}
?>
<?php
}else{
header('Location:login.php');
}
?>
<a href="logout.php">Me deconnecter</a>
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