Call to undefined function mysql_connect()

$username = $_POST;$password = $_POST;$gender = $_POST;$email = $_POST;$phoneCode = $_POST;$phone = $_POST;if (!empty($username) || !empty($password) || !empty($gender) || !empty($email) || !empty($phoneCode) || !empty($phone)) { $host = "localhost"; $dbUsername = "root"; $dbPassword = ""; $dbname = "youtube"; //create connection $conn = new mysqli($host, $dbUsername, $dbPassword, $dbname); if (mysqli_connect_error()) { die('Connect Error('. mysqli_connect_errno().')'. mysqli_connect_error()); } else { $SELECT = "SELECT email From register Where email = ? Limit 1"; $INSERT = "INSERT Into register (username, password, gender, email, phoneCode, phone) values(?, ?, ?, ?, ?, ?)"; //Prepare statement $stmt = $conn->prepare($SELECT); $stmt->bind_param("s", $email); $stmt->execute(); $stmt->bind_result($email); $stmt->store_result(); $rnum = $stmt->num_rows; if ($rnum==0) { $stmt->close(); $stmt = $conn->prepare($INSERT); $stmt->bind_param("ssssii", $username, $password, $gender, $email, $phoneCode, $phone); $stmt->execute(); echo "New record inserted sucessfully"; } else { echo "Someone already register using this email"; } $stmt->close(); $conn->close(); }} else { echo "All field are required"; die();}?>index.php<?phpinclude_once('connection.php');$query="select * from register";$result=mysql_query($query);?><!DOCTYPE html><html><?php while($rows=mysql_fetch_assoc($result)) { ?> <?php echo $rows; ?> <?php echo $rows; ?> <?php echo $rows; ?> <?php echo $rows; ?> <?php } ?>