I went to update my Fedora Core 5 machine today, and received the following error:
[root ~] yum check-update
Loading "installonlyn" plugin
Setting up repositories
core [1/3]
Cannot find a valid baseurl for repo: core
Error: Cannot find a valid baseurl for repo: core
I tried running yum clean all but it didn't help. After looking around I found that all of my repo files in /etc/yum.repos.d/ had their baseurl values commented out. Don't ask me how that happened, I have no idea. Simply un-commenting all the baseurl values in all my repo files fixed the problem, and yum is once again running as expected. So if you run into this problem, make sure everything is set up right in /etc/yum.repos.d/*.repo

The instructions for doing so (here) seemed straightforward enough, but I ran into a few problems. So here's how I got PDFLib working with my PHP install on Fedora Core 4.

First we need to build PDFlib Lite from Source. Download it from here. Unpack it, then:
./configure
make
make install
Easy enough. Now:
yum install automake
yum install php-devel
pecl install pdflib
(Note: php-devel is required because we need phpize which is used by the pecl command.)

The PECL install will ask you a question: "Path to PDFlib installation?" This is where I ran into problems. You must put /usr/local/ and NOT /usr/local/include because the script is hardcoded to look inside the include directory automatically. Once PECL finds pdflib.h it will continue and finish compiling our shared object, pdf.so. The script will install it for us in /usr/lib/php/modules/
Now we just have to tell PHP to load the SO. Open up php.ini and add the following line:
extension=pdf.so
Save & exit, and restart your Apache with apachectl graceful. Now browse to a file containing the phpinfo() function and check to see if PDFlib is now active. If you see it, you're good to go!

NOTE: I ran into a problem with RPM GPG Keys at the automake install step. I was getting the following error:
warning: rpmts_HdrFromFdno: Header V3 DSA signature: NOKEY, key ID db42a60e
public key not available for autoconf-2.59-5.noarch.rpm
Retrieving GPG key from file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora
The GPG key at file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora (0x4F2A6FD2)
is already installed but is not the correct key for this package.
Check that this is the correct key for the "Fedora Core 4 - i386 - Base" repository.
To get around this we need to import all the keys by run the following command:
rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY*
I found this solution in this thread.

Adding an additional IP or IPs to your network card in Linux is easy. Here's how I did it in my Fedora Core 4 installation:
cd /etc/sysconfig/network-scripts/
cp ifcfg-eth0 ifcfg-eth0:0
Now you will need to open your newly created ifcfg-eth0:0 file in your favorite editor and modify it to fit your needs. The two lines you MUST change are the following, the rest are optional:
DEVICE=eth0:0
IPADDR=192.x.x.x
Of course you will want to fill in the IPADDR value to fit your needs.

Now, to make the changes take effect, you will need to bring your new IP up by issuing the following command:
./ifup eth0:0
Or if you want you can just restart the entire network:
/etc/rc.d/init.d/network reload
There you go! Run ifconfig and you should see your new IP assigned to eth0:0. It should look something like this:
eth0:0 Link encap:Ethernet HWaddr 00:D0:B7:B7:XX:XX
inet addr:192.168.1.XX Bcast:192.168.1.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
You can add even more IPs by repeating this process and incrementing the value of eth0:0, for example you can add eth0:1, eth0:2, etc.