From my research:

• Filter is enabled by default as of 5.2.0 - http://php.net/manual/en/filter.installation.php
• GD must be installed - http://php.net/manual/en/image.installation.php
• json is enabled by default as of 5.2.0 - http://php.net/manual/en/json.installation.php
• mysql is enabled by default as of 5.0.0 and updated in 5.3.0 - http://php.net/manual/en/mysqli.installation.php
• mbstring must be installed if you're using multi-byte strings - http://php.net/manual/en/mbstring.installation.php
• session is enabled by default - http://php.net/manual/en/session.installation.php
• xml is enabled by default - http://php.net/manual/en/xml.installation.php
• zlib must be installed - http://php.net/manual/en/zlib.installation.php

I'll add GD, mbstring (optional), and zlib to the requirements list. Thanks.

Updated: https://github.com/web2project/web2project#requirements

Thanks, I understood your point. But actually the defaults depend on a way how php was installed. So it would be great to have a full list of extensions documented. Ideally, installer should check and display status of each required and optional extension as roundcube installer does.

For instance php related FreeBSD ports/packages:

• lang/php56 - php itself (all extensions are disabled)
• separate ports for each extension
• lang/php56-extensions - metaport - default extensions:
  ctype, dom, filter, hash, iconv, json, pdo, pdo_sqlite, phar, posix,
  session, simplexml, sqlite3, tokenizer, xml, xmlreader and xmlwriter.

That makes sense.

Any chance you could write something along the lines of this:
https://github.com/web2project/web2project/blob/master/install/install/check.php#L41
and do a pull request?

Ok, I'm on it.
Is the extensions list I mentioned at the top complete or I missed something?

I'm not 100% sure it's complete but it's enough to start. :)