These are not actual extensions. They are part of the PHP core and cannot be left out of a PHP binary with compilation options.
Arrays
Classes/Objects
Date/Time
Directories
Error Handling
Program execution
Filesystem
Function Handling
i18n
PHP Options/Info
Mail
Math
Misc.
Network
Output Control
POSIX Regex
Sessions
Streams
Strings
Tokenizer
Unicode
URLs
Variable handling
These extensions are bundled with PHP.
Apache
BC Math
Bzip2
Calendar
COM
Ctype
cURL
DBA
dBase
DOM
.NET
Exif
FrontBase
FDF
FTP
Gettext
GMP
Hash
Firebird/InterBase
iconv
Informix
GD
IMAP
JSON
LDAP
libxml
Multibyte String
Mcrypt
Mhash
Mimetype
Ming
mSQL
Mssql
MySQL
Mysqli
Ncurses
NSAPI
Object Aggregation
OCI8
OpenSSL
PCNTL
PCRE
PDO
MS SQL Server (PDO)
Firebird/Interbase (PDO)
MySQL (PDO)
Oracle (PDO)
ODBC and DB2 (PDO)
PostgreSQL (PDO)
SQLite (PDO)
PostgreSQL
POSIX
Pspell
Readline
Recode
Semaphore
Shared Memory
SimpleXML
SNMP
SOAP
Sockets
SPL
SQLite
SVN
Sybase
Tidy
ODBC
WDDX
XML Parser
XMLReader
XML-RPC
XSL
XSLT
Zip
Zlib
In order to compile these extensions, external libraries will be needed.
BBCode
Crack
DB++
dbx
DOM XML
Enchant
FAM
Fileinfo
FriBiDi
GeoIP
haru
Hyperwave
Hyperwave API
IBM DB2
ImageMagick
Ingres II
Java
MaxDB
Memcache
mnoGoSearch
Msession
Newt
oggvorbis
OpenAL
Ovrimos SQL
Paradox
PDF
IBM (PDO)
Informix (PDO)
PS
qtdom
Radius
Rar
Session PgSQL
Sphinx
SSH2
SWF
Swish
vpopmail
xattr
xdiff
YAZ
These extensions are available from » PECL. More PECL extensions exist but they are not documented in the PHP manual yet.
APC
APD
bcompiler
Classkit
Cyrus
Direct IO
Expect
filePro
Filter
GnuPG
HTTP
ID3
IIS
KADM5
LZF
Mailparse
MCVE
Gopher
YP/NIS
Lotus Notes
Parsekit
Phar
Printer
RPM Reader
runkit
SAM
SCA
SDO
SDO DAS XML
SDO-DAS-Relational
Statistics
TCP
W32api
win32ps
win32service
XMLWriter