What are PHP extensions and libraries WP needs and/or uses?

Codex loosely only mentions PHP version in server requirements, however PHP can be configured in pretty wide range of excluding/including different parts.

What are extensions/libraries that:

1. are required for WP to work at all? MySQL…
2. are required for specific functions? GD ?…
3. are preferred for best operation of specific functions? cURL…
4. are commonly used by plugins? Zip…

I gave this some more thought and given size of WP code base (including bundled libraries) it doesn’t seem very realistic to compile such list by hand (and repeat it for every new version – meh).

I looked up appropriate static code analysis tool – PHP_CompatInfo and after some tinkering generated following report of extensions used by core (version scanned 3.3.1):

• cURL – 127 uses (requires libcurl)

  • HTTP API (class WP_Http_curl)
  • url_is_accessable_via_ssl()
  • SimplePie (overridden with class WP_SimplePie_File)
  • GoogleSpell (from TinyMCE package, is not used?)

• Date/Time – 367 uses

• DOM – 6 uses (requires libxml)

  • iis7_rewrite_rule_exists()
  • iis7_delete_rewrite_rule()
  • iis7_add_rewrite_rule()
  • saveDomDocument()

• POSIX Regex – 23 uses

• Filter – 2 uses

  • class PHPMailer->ValidateAddress() (optional)

• FTP – 72 uses

  • class ftp_base
  • class ftp (pure and sockets versions)
  • class WP_Filesystem_FTPext
  • class WP_Filesystem_ftpsockets

• GD – 56 uses

  • wp-admin\includes\image-edit.php
  • wp-admin\includes\image.php
  • wp-includes\media.php

• Hash – 6 uses

  • wp-includes\pluggable.php multiple uses (optional – fallback in wp-includes\compat.php)

• iconv – 5 uses

  • class SimplePie (optional)
  • wp_check_invalid_utf8() (optional)
  • wp-mail.php (optional)

• JSON – 20 uses

  • optional, fallbacks in wp-includes/compat.php

• libxml – 4 uses

  • class WP_oEmbed->_parse_xml() (optional)
  • SimplePie

• Multibyte String – 29 uses

  • some fallback in wp-includes/compat.php

• MySQL – 60 uses

  • class wpdb
  • class SimplePie_Cache (overridden with class WP_Feed_Cache)

• OpenSSL – 4 uses

  • class PHPMailer

• PCRE – 743 uses

• SimpleXML – 1 uses

  • class WP_oEmbed (seems optional)

• Sockets – 64 uses

  • class ftp (sockets implementation)

• SPL – 3 uses

• Tokenizer – 3 uses

  • wp_doc_link_parse() (optional)

• XML Parser – 89 uses

• XMLReader – 1 uses

  • SimplePie (seems optional)

• Zlib – 30 uses