Here is a brief list of php4 modules that come standard with our shared/sdx/reseller servers:
bcmath, calendar, ctype, curl, dom, domxml, exif, ftp, gd (with FreeType), gettext, iconv, imap, mbstring, mbregex, mcrypt, mhash, mm, mysql, mysqli, openssl, overload, pcre, pdflib, pear, posix, pspell, sablot, session, sockets, standard, tokenizer, wddx, xml, xmlrpc, xslt, zip, zlib – and Zend Optimizer
php5 modules:
bcmath, calendar, ctype, curl, date, dom, exif, filter, ftp, gd (with FreeType), gettext, hash, iconv, imap (with ssl), json, libxml, mbstring, mcrypt, mhash, mysql, mysqli, openssl, pcre, PDO, pdo_mysql, pdo_sqlite, posix, pspell, Reflection, session, SimpleXML, soap, sockets, SPL, SQLite, standard, tidy 2, tokenizer, wddx, xml, xmlreader, xmlrpc, xmlwriter, xsl, Zend Optimizer, zip, zlib.
If you need any other module that is officially supported by php, or one of the above module is missing from your server, please open a support ticket and we can evaluate if it is possible to install it.