For Gentoo

/etc/portage/package.mask

>=dev-lang/php-5

/etc/portage/package.unmask

=dev-lang/php-4.4.8_pre20070816
=dev-php4/phpunit-1.3.2

/etc/portage/package.use

=dev-lang/php-4* -* cli cgi apache2 ctype curl expat fastbuild force-cgi-redirect ftp gd iconv ipv6 ldap memlimit mysql nls pcre pic posix session sockets ssl tokenizer truetype unicode xml xsl zlib
media-gfx/imagemagick lcms
media-video/ffmpeg encode