Версия языка PHP 5.1 доступна на всех тарифах хостинга cPanel hosting.
Как изменить версию PHP написано в статье Выбор версии PHP.

PHP 5.1 (количество модулей 93): bcmath, big_int, bitset, bz2, bz2_filter, calendar, coin_acceptor, crack, ctype, curl, date, dba, dbase, dom, doublemetaphone, exif, ftp, gd, geoip, gettext, gmagick, gmp, gnupg, haru, hash, huffman, iconv, idn, igbinary, imagick, imap, inclued, inotify, interbase, ioncube_loader, ioncube_loader_4, ldap, libxml, lzf, mbstring, mcrypt, memcache, msgpack, mysql, mysqli, ncurses, odbc, openssl, pcntl, pcre, pdo, pdo_firebird, pdo_mysql, pdo_oci, pdo_odbc, pdo_pgsql, pdo_sqlite, pgsql, posix, pspell, quickhash, radius, readline, redis, reflection, session, shmop, simplexml, snmp, soap, sockets, sourceguardian, spl, ssh2, standard, stats, stem, sybase_ct, sysvmsg, sysvsem, sysvshm, tidy, timezonedb, tokenizer, translit, wddx, xdebug, xml, xmlreader, xmlrpc, xmlwriter, xsl, zlib