Impossible de faire enregistrement Mysql PDO sous Mac

select max(id)+1

<?php 
 
 if(isset($_POST['nom']) && isset($_POST['prenom'])) {
  
 $_POST['nom'] = stripslashes($_POST['nom']);
  $_POST['prenom'] = stripslashes($_POST['prenom']);
  $_GET['class'] = stripslashes($_GET['class']);
  $_GET['cycle'] = stripslashes($_GET['cycle']);
  $_GET['serie'] = stripslashes($_GET['serie']);
     
 
$dn2 = $bdd->prepare('SELECT max(ideleve) FROM eleve');
$dn2->execute();
$id = $dn2+1;


$req = $bdd->prepare('INSERT INTO eleve(nom, prenom, cycle, class, serie, id, ideleve)  VALUES(:nom, :prenom, :cycle, :class, :serie, :id, :ideleve)');
     
$valeurs = ['nom' => $_POST['nom'], 'prenom' => $_POST['prenom'], 'cycle' => $_GET['cycle'], 'class' => $_GET['class'], 'serie' => $_GET['serie'], 'id' => $_SESSION['id'], 'ideleve' => $id];      
$req->execute($valeurs);     

}
 ?>  

 <?php  
     
  if(isset($_POST['nomat'])) {
 
 $_POST['nomat'] = stripslashes($_POST['nomat']);
 $_GET['class'] = stripslashes($_GET['class']);
 $_GET['cycle'] = stripslashes($_GET['cycle']);
 $_GET['serie'] = stripslashes($_GET['serie']);
     

 
$dn2 = $bdd->prepare('SELECT max(idmat) FROM matiere');
$dn2->execute();
$id = $dn2+1;   
 
 //On enregistre les informations dans la base de donnee
     
$requet = $bdd->prepare('INSERT INTO matiere(nomat, class, cycle, idmat, id)  VALUES(:nomat, :class, :cycle, :idmat, :id)');
     
$valors = ['nomat' => $_POST['nomat'], 'class' => $_GET['class'], 'cycle' => $_GET['cycle'],'idmat' => $id, 'id' => $_SESSION['id']]; 

try{
   
$requet->execute($valors);
 
}catch(Exception $e){
  // en cas d'erreur :
   echo " Erreur ! ".$e->getMessage();
   echo " Les datas : " ;
  print_r($valors);
 
} 
 
     
 }    
  ?>

$dn2 = $bdd->prepare('SELECT max(idmat) FROM matiere');
$dn2->execute();
$id = $dn2+1;

$result = $dn2->fetchColumn();
$id = $result+1;

print_r( $_POST ) ;

<?php
session_start();
//début de ton fichier php .... ça veut dire .. DEBUT !!!:
error_reporting ( E_ALL );
ini_set ( 'display_errors', TRUE );
ini_set ( 'display_startup_errors', TRUE );

require_once "connexion.php";

$nomat = !empty ( $_POST['nomat'] ) ? $_POST['nomat'] : NULL;
$coef = !empty ( $_POST['coefficient'] ) ? $_POST['coefficient'] : NULL;
$class = !empty ( $_GET['class'] ) ? $_GET['class'] : NULL;
$cycle = !empty ( $_GET['cycle'] ) ? $_GET['cycle'] : NULL;
$serie = !empty ( $_GET['serie'] ) ? $_GET['serie'] : NULL;
$iduser = !empty ( $_SESSION['iduser'] ) ? $_SESSION['iduser'] : NULL;

if ( $nomat ) {
  $sql = 'SELECT idmat FROM matiere';
  try {
    $req = $bdd->prepare ( $sql );
    $req->execute ( $valeurs );
    $result = $req->fetchColumn ();
    $id = $result + 1;
    $sql = 'INSERT INTO matiere(nomat, coefficient, `class`, `cycle`, `serie`, idmat,iduser)
                    VALUES(:nomat, :coefficient, :class, :cycle, :serie, :idmat, :iduser)';
    $datas = array(':nomat' => $nomat, ':coefficient' => $coef, ':class' => $class, ':cycle' => $cycle, ':serie' => $serie, ':idmat' => $id, ':iduser' => $iduser);
    try {
      $req = $bdd->prepare ( $sql );
      if ( $req->execute ( $datas ) ) {
        echo "<br> Insertion réalisée avec succès ! <br>";
      } else {
        echo "<br> Erreur lors de l'enregistrement... <br>";
        print_r ( $datas );
        exit;
      }
    } catch ( Exception $e ) {
      // en cas d'erreur :
      echo "<br> Erreur lors de l'enregistrement en BDD ! " . $e->getMessage ();
      print_r ( $valeurs );
      exit;
    }
  } catch ( Exception $e ) {
    // en cas d'erreur :
    echo " Erreur ! " . $e->getMessage ();
    print_r ( $valeurs );
    exit;
  }
}
?>
<div class="site-section bg-light">
  <div class="container">
    <div class="row">

      <div class="col-md-12 col-lg-8 mb-5">
        <?php 
        echo '<p class="mb-0 font-weight-bold">
              <h3 class="mb-0">' .$class .'</h3>';
        echo $serie . ' </p><br>';
        ?>
        <form action="" method="post" class="p-5 bg-white">
          <div class="row form-group">
            <div class="col-md-12 mb-3 mb-md-0">
              <label class="font-weight-bold" for="nomat">Nom Matière</label>
              <input name="nomat" type="text"  class="form-control" placeholder="" >
            </div>
            <div class="col-md-6 col-lg-3 mb-3 mb-lg-0">
              <label class="font-weight-bold" for="coefficient">Coefficient</label>
              <select name="coefficient" id="coefficient" class="form-control">
                <?php
                 for($i=1;$i<=10;$i++){
                  echo '<option value="'.$i.'">'.$i.'</option>';
                 }
                ?>
              </select>

            </div> 
            <div class="col-md-6 col-lg-3 mb-3 mb-lg-0">
            </div>
          </div>
          <div class="row form-group">
            <div class="col-md-12">
              <input type="submit" value="Enregistrez" class="btn btn-primary  py-2 px-4">
            </div>
          </div>
        </form>
      </div>