<form method="GET" action="del-post.php?delete_id=<?php echo $article_info['article_id'];?>">
<button type="submit" class="btn btn-danger btn-lg" data-toggle="modal" data-target="#delModal">Удалить</button>
</form>
<form method="GET" action="del-post.php">
<input type="hidden" name="delete_id" value="<?php echo $article_info['article_id'];?>">
<button type="submit" class="btn btn-danger btn-lg" data-toggle="modal" data-target="#delModal">Удалить</button>
</form>
<a href="del-post.php?delete_id=<?php echo $article_info['article_id'];?>">
<button type="submit" class="btn btn-danger btn-lg" data-toggle="modal" data-target="#delModal">Удалить</button>
</a>