Looking for a SEO friendly Bulleting Board / Forum

A quick, practical summary that complements the replies from @obiztek, @Dani and @iwonder:

For SEO you’re aiming for stable, human-readable permalinks, unique page titles/meta descriptions, and crawlable content. The literal file extension (.html vs .php) is mostly cosmetic — search engines care far more that the URL is stable, contains useful words (a slug) and that duplicate URLs are handled correctly. Build your URL strategy around an ID+slug pattern (so each thread has a single canonical address), then enforce that pattern with either the forum’s permalink options or with server rewrites and 301s.

Essential checklist (prioritize these over the “.html” look):

• Stable permalink that includes an identifier and a readable slug.
• Unique <title> and meta description for each thread.
• rel=canonical or 301 redirects to avoid duplicate-content from querystrings/pagination.
• An XML sitemap for threads and regular lastmod updates.
• Good robots rules and no JS-only navigation for thread discovery.

Practical example (Apache .htaccess) — maps friendly URL to the thread script:

RewriteEngine On
RewriteBase /forums/
# friendly: /forums/topic-title_t123.html  -> showthread.php?t=123
RewriteRule ^(.+)_t([0-9]+)\.html$ showthread.php?t=$2 [L,QSA]

Also canonicalize: when someone hits showthread.php?t=123 have your script generate (or 301-redirect to) the preferred slugged URL so search engines index only that form. A small PHP redirect inside your thread loader will do that safely.

Pick software based on support and extensibility: open-source boards usually have plugins for permalinks; commercial packages give official support and paid SEO plugins. Whatever you choose, test crawlability (use a crawler or Google Search Console), add sitemaps, and implement caching — these steps will have a bigger impact than chasing a specific file extension.