<form action="createnew.php" method="post">
	<label for="title">Название</label>
	<input type="text" name="title" id="title">
	<select name="navigation" id="">
		<option selected>Категория</option>
		<?php foreach ($navigation as $nav): ?>
		<option value="<?=$nav['id'];?>"><?=$nav['title'];?></option>
		<?php endforeach;?>
	</select>
	<textarea name="content" rows="10"></textarea>
	<button name="add-post" type="submit">Запостить</button>
</form>

// Добавление новости
if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_POST['add-post'])) {

	// Перемнные с формы
	$nav = $_POST['navigation'];
	$title = $_POST['title'];
	$content = $_POST['content'];

	try {
		global $connectdb;
		$data = [
			'navigation' => $nav,
			'title' => $title,
			'content' => $content,
			'datarelease' => date("Y-m-d")
		];

		$query = $connectdb->prepare("INSERT INTO `articles` (navigation, title, content, datarelease) values (:navigation, :title, :content, :datarelease)");
		$query->execute($data);
	} catch (PDOException $e){
		print "Ошибка! " . $e->getMessage() . "<br/>";
	}
}