Ignore postname in permalink

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Being in wp-admin/options-permalink.php (Permalinks) you can easily set this using Custom Structure:

/%post_id%/%postname%/

custom permalink structure

So you can browse the Hello World post with:

http://example.com/1/hello-world/

But if you strip out the postname from the URL like below:

http://example.com/1/

You can still access the Hello World post.

CAUTION Changing permalink in any live site may cause bad effect in SEO, and cause huge amount of 404 pages without proper redirection. So just think about it before switching to any changes in permalink.