This installation of PHP will need some extra features, like maybe SOAP or XML functionality. For XML you need libxml2-dev. On a Debian machine this can be installed with:

# apt-get install libxml2-dev
# apt-get install openssl libssl-dev

Even if you already had that library, it wouldn't hurt to see if it has an upgrade available.

cd into the php directory:

$ cd ../php-5.1.2

Configure PHP with GD, XML, OpenSSL .. and more!

$ ./configure \
--with-apache=../apache_1.3.34 \
--with-mysql=/usr/local/mysql \
--with-gd \
--with-jpeg-dir \
--enable-jpeg \
--with-xml \
--with-zlib=/usr \
--with-mcrypt \
--with-dom-xslt \
--with-dom-exslt \
--enable-gd-native-ttf \
--with-ttf \
--with-openssl \
--enable-ftp \
--enable-png \
--enable-soap
		

If you get an error, it's most likely you are missing some development libraries. If you get a "Thank you for using PHP." message, you can continue...

# make
# make install
# cp php.ini-dist /usr/local/lib/php.ini