If you decided to create this on your own then... the most efficient way to do this would be to process it once when the author submits the article.
First break up the db into two tables: articles and pages.
[THIS IS NOT Actual Code and the DB can be optimized or implemented in a better way]
Articles
-id : int
-title : varchar(255)
-author : varchat(255)
-description : varchar(255)
Pages
-id : int
-article_id : int
-page_number : int
-content : text
Then just search for
SELECT * FROM Pages where article_id = $articleID AND page_number = $pagenum
The Benefits:
Less data is being retrieved from MySQL and PHP doesn't need to parse the text for [nextpage] every time someone is viewing a page in a article.
Let me know if this helps.