Test

<?php
header('Content-Type: text/html; charset=utf-8');
include('./include/mysql.php');
$error = null;

if (isset($_POST['username'])) {
  if (!empty($_POST['username'])) {
    if (!empty($_POST['password'])) {
      if (!empty($_POST['confirm_password'])) {
        if (!empty($_POST['mail'])) {
          if ($_POST['password'] == $_POST['confirm_password']){
            $username = $_POST['username'];
            $password = $_POST['password'];
            $mail = $_POST['mail'];
            $ip = $_SERVER["REMOTE_ADDR"];
            $date = date('Y-m-d');

            if (strlen($username) >= 3){
              if (strlen($username) < 15) {
                if (preg_match('[a-zA-Z0-9-_]', $username)){
                  echo "Ok";
                  if (strlen($password) >= 6){
                    if(strlen($mail) < 254){
                      $nonASCII='ďđēĕėęěĝğġģĥħĩīĭįıĵķĺļľŀłńņňʼnŋōŏőoeŕŗřśŝsťŧ';
                      $nonASCII.='ďđēĕėęěĝğġģĥħĩīĭįıĵķĺļľŀłńņňʼnŋōŏőoeŕŗřśŝsťŧ';
                      $nonASCII.='ũūŭůűųŵŷźżztșțΐάέήίΰαβγδεζηθικλμνξοπρςστυφ';
                      $nonASCII.='χψωϊϋόύώабвгдежзийклмнопрстуфхцчшщъыьэюяt';
                      $nonASCII.='ἀἁἂἃἄἅἆἇἐἑἒἓἔἕἠἡἢἣἤἥἦἧἰἱἲἳἴἵἶἷὀὁὂὃὄὅὐὑὒὓὔ';
                      $nonASCII.='ὕὖὗὠὡὢὣὤὥὦὧὰάὲέὴήὶίὸόὺύὼώᾀᾁᾂᾃᾄᾅᾆᾇᾐᾑᾒᾓᾔᾕᾖᾗ';
                      $nonASCII.='ᾠᾡᾢᾣᾤᾥᾦᾧᾰᾱᾲᾳᾴᾶᾷῂῃῄῆῇῐῑῒΐῖῗῠῡῢΰῤῥῦῧῲῳῴῶῷ';
                      $syntaxe = "#^[[:alnum:][:punct:]]{1,64}@[[:alnum:]-.$nonASCII]{2,253}\.[[:alpha:].]{2,6}$#";

                      if(preg_match($syntaxe,$mail)){
                        $password = sha1($_POST['password']);
                        $verification = $bdd->prepare('SELECT COUNT(*) FROM users WHERE username = ?');
                        $verification->execute(array($username));
                        if ($verification->fetchColumn() <=0) {
                          $verification = $bdd->prepare('SELECT COUNT(*) FROM users WHERE mail = ?');
                          $verification->execute(array($mail));
                          if ($verification->fetchColumn() <=0) {
                            $register = $bdd->prepare('INSERT INTO users(username, password, mail, register_date, grade, last_ip) VALUES(?, ?, ?, ?, ?, ?)');
                            $register->execute(array($username, $password, $mail, $date, "0", $ip));
                            $_SESSION['user'] = $username;
                          }else{ $error = "13"; }//mail_utilisé
                        }else{ $error = "12"; }//pseudo_utilisé
                      }else{ $error = "11"; }//adresse pas bonne
                    }else{ $error = "10"; }//adresse trop longue
                  }else{ $error = "9"; }//mdp trop court
                }else{ $error = "8"; }//username pas bon
              }else{ $error = "7"; }//user trop long
            }else{ $error = "6"; }//user trop court
          }else{ $error = "5"; }//mdp_correspondant
        }else{ $error = "4"; }//mail
      }else{ $error = "3"; }//confirm
    }else{ $error = "2"; }//password
  }else{ $error = "1"; }//username
}
?>