try
{
	$bdd = new PDO('mysql:host=localhost;dbname=rmxosdb', 'root', '');
}
catch(Exception $e)
{
        die('Erreur : '.$e->getMessage());
}

$bdd->exec('INSERT INTO user_data(user_id, notrade, lastlogin, guild_id) VALUES(NULL, DEFAULT, NOW(), NULL)');

echo 'Inscription validée';

   CREATE TABLE IF NOT EXISTS `user_data` (
  `user_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `notrade` tinyint(1) NOT NULL DEFAULT '0',
  `lastlogin` datetime NOT NULL,
  `guild_id` int(10) unsigned DEFAULT '0',
  PRIMARY KEY (`user_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 ;

          $insertion = 
		 "INSERT INTO users (user_id, username, password, mail, naissance, inscription)
              VALUES (NULL, '$pseudo', '$mdp', '$mail', '$date_naissance', NOW())";
		 "INSERT INTO user_data (user_id)
	      VALUES (NULL)";

CREATE TABLE user(
        userId      int(11) Auto_increment  NOT NULL ,
        username    Varchar(25) NOT NULL ,
        passwd      Varchar(128) NOT NULL ,
        mail        Varchar(255) NOT NULL ,
        naissance   Date NOT NULL ,
        inscription Date NOT NULL ,
        lastLogin   Datetime,
        PRIMARY KEY (userId)
)ENGINE=InnoDB;

CREATE TABLE guild(
        guildId int(11) Auto_increment  NOT NULL ,
        nom     Varchar(25) NOT NULL ,
        PRIMARY KEY (guildId)
)ENGINE=InnoDB;

CREATE TABLE Perso(
        persoId       int(11) Auto_increment  NOT NULL ,
        nom           Varchar(25) NOT NULL ,
        notrad        Int NOT NULL default 0 ,
        lastLogin     Datetime,
        userId_user   Int NOT NULL ,
        guildId_guild Int NOT NULL ,
        PRIMARY KEY (persoId),
        UNIQUE (nom)
)ENGINE=InnoDB;

ALTER TABLE Perso ADD CONSTRAINT FK_Perso_userId_user FOREIGN KEY (userId_user) REFERENCES user(userId);
ALTER TABLE Perso ADD CONSTRAINT FK_Perso_guildId_guild FOREIGN KEY (guildId_guild) REFERENCES guild(guildId);

Je pense que tu as laissé passé une petite erreur dans '".$date_naissance."' avec le point une case trop loin et aussi $idUtilisateur qui forcement n'existe pas chez moi.. est il égal à $user_id?

$insertion = "INSERT INTO users (user_id, username, password, mail, naissance, inscription) VALUES (NULL, '$pseudo', '$mdp', '$mail', '$date_naissance', NOW())";
$insertion = "INSERT INTO user_data (user_id, notrade, lastlogin, guild_id) VALUES (".$idUtilisateur.", '0', NOW(), '0')";

$insertion = "INSERT INTO users ( username, password, mail, naissance, inscription) VALUES ( '".$pseudo."', '".$mdp."', '".$mail."', '."$date_naissance."', NOW())";

$insertion = "INSERT INTO user_data (user_id, notrade, lastlogin, guild_id) VALUES (".$idUtilisateur.", 0, NOW(), 0)";

CREATE TABLE IF NOT EXISTS `user_data` (
  `user_id` int(11) UNSIGNED NOT NULL AUTO_INCREMENT,
  `notrade` tinyint(1) NOT NULL DEFAULT '0',
  `lastlogin` datetime NOT NULL,
  `guild_id` int(10) UNSIGNED DEFAULT NULL,
  PRIMARY KEY (`user_id`),
  KEY `guild_id` (`guild_id`)
)

CREATE TABLE IF NOT EXISTS `user_data` (
  `user_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `notrade` tinyint(1) NOT NULL DEFAULT '0',
  `lastlogin` datetime NOT NULL,
  `guild_id` int(10) unsigned DEFAULT NULL,
  PRIMARY KEY (`user_id`),
  KEY `guild_id` (`guild_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=74 ;

$insertion = "INSERT INTO users (user_id, username, password, mail, naissance, inscription) VALUES (NULL, '$pseudo', '$mdp', '$mail', '$date_naissance', NOW())";
$insertion = "INSERT INTO user_data (user_id, notrade, lastlogin, guild_id) VALUES (NULL, '0', NOW(), '0')";