/etc/apt/sources.list
# PHP5
#deb http://people.debian.org/~dexter php5 sarge
deb http://people.debian.org/~dexter php5.1 sarge
Install
# aptitude install php5.1 php5.1-gd php5.1-mbstring
$ COLUMNS=112 dpkg -l |grep php5.1
ii libapache2-mod-php5.1 5.1.4-0.1~sarge1 HTML-embedded scripting language (apache 2.0 module)
ii php5.1 5.1.4-0.1~sarge1 server-side, HTML-embedded scripting language (meta-package)
ii php5.1-cgi 5.1.4-0.1~sarge1 HTML-embedded scripting language (CGI binary)
ii php5.1-cli 5.1.4-0.1~sarge1 command-line interpreter for the PHP 5.1 scripting language
ii php5.1-common 5.1.4-0.1~sarge1 common files for packages built from the php5.1 source
ii php5.1-gd 5.1.4-0.1~sarge1 GD module for PHP 5.1
ii php5.1-mbstring 5.1.4-0.1~sarge1 mbstring module for PHP 5.1
ii php5.1-mysql 5.1.4-0.1~sarge1 MySQL module for PHP 5.1
ii php5.1-session 5.1.4-0.1~sarge1 session module for PHP 5.1
ii php5.1-sqlite 5.1.4-0.1~sarge1 SQLite module for PHP 5.1