inputs:
$_POST['phrase'] = "H300"
$_POST['description'] = "Don't use with water."
-------------------------------------
$new_phrase = $_POST['phrase'];
$new_description = $_POST['description'];

$sql = "INSERT INTO ghs_phrases (phrase, description) VALUES(:phrase, :description1) ON DUPLICATE KEY UPDATE description=:description2";
$query = $pdo->prepare($sql);
$query->execute(array('phrase' => $new_phrase, 'description1' => $new_description, 'description2' => $new_description));