PDO: Insertion ne se fait pas

Bonjour et meilleurs voeux à tous.
J'ai une table vide avec la structure ci-dessous:

Code:

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SET SQL_MODE = "NO_AUTO_VALUE_ON_ZERO";
START TRANSACTION;
SET time_zone = "+00:00";
 
DROP TABLE IF EXISTS `dat_books`;
CREATE TABLE IF NOT EXISTS `dat_books` (
  `id` int(10) UNSIGNED NOT NULL AUTO_INCREMENT,
  `title` varchar(50) NOT NULL,
  `id_author` int(10) UNSIGNED DEFAULT NULL,
  `id_publisher` int(10) UNSIGNED DEFAULT NULL,
  `id_place` int(10) UNSIGNED DEFAULT NULL,
  `id_public` int(10) UNSIGNED DEFAULT NULL,
  `id_theme` int(10) UNSIGNED DEFAULT NULL,
  `format` varchar(30) DEFAULT NULL,
  `comment` text NOT NULL,
  `update_date` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`),
  KEY `id_auteur` (`id_author`),
  KEY `id_editeur` (`id_publisher`),
  KEY `id_place` (`id_place`),
  KEY `id_theme` (`id_theme`),
  KEY `id_public` (`id_public`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
 
 
ALTER TABLE `dat_books`
  ADD CONSTRAINT `dat_books_ibfk_1` FOREIGN KEY (`id_author`) REFERENCES `lst_authors` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
  ADD CONSTRAINT `dat_books_ibfk_2` FOREIGN KEY (`id_publisher`) REFERENCES `lst_publishers` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
  ADD CONSTRAINT `dat_books_ibfk_3` FOREIGN KEY (`id_place`) REFERENCES `lst_places` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
  ADD CONSTRAINT `dat_books_ibfk_4` FOREIGN KEY (`id_theme`) REFERENCES `lst_themes` (`id`) ON DELETE SET NULL ON UPDATE CASCADE,
  ADD CONSTRAINT `dat_books_ibfk_5` FOREIGN KEY (`id_public`) REFERENCES `lst_public` (`id`) ON DELETE SET NULL ON UPDATE CASCADE;
COMMIT;

J'ai la fonction PDO suivante:

Code:

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function insertBook(array $data): false|string
{
	$db = dbConnect();
	extract($data);
	var_dump($data);
 
	$sql = <<<SQL
INSERT IGNORE INTO dat_books
(title, id_author, id_publisher, id_theme, id_public, id_place, format, comment)
VALUES(:title, :author, :publisher, :theme, :public, :place, :format, :comment) 
 
SQL;
	$params = [':title'=>$title, ':author'=>$author, ':publisher'=>$publisher, ':theme'=>$theme, ':public'=>$public, ':place'=>$place, ':format'=>$format, ':comment'=>$comment, ];
	$stmt = $db->prepare($sql);
	$stmt->execute($params);
	return $db->lastInsertId();   // retourne 0
}

Cette fonction ne renvoie aucune erreur MySQL.
La ligne 5 renvoie ceci:

array (size=8)
  'title' => string 'Coucou' (length=6)
  'author' => string '1' (length=1)
  'publisher' => string '1' (length=1)
  'theme' => string '1' (length=1)
  'public' => string '1' (length=1)
  'place' => string '' (length=0)
  'format' => string '' (length=0)
  'comment' => string '' (length=0)

Bien que la table soit vide, l'insertion ne se fait pas mais l'auto_incrément augmente.
EDIT: Le mot clef IGNORE est important et j'ai ajouté une clef unique UNIQUE KEY `title` (`title`,`id_author`,`id_publisher`)