yum Download All Source Packages (SRPM) From RedHat / CentOS WebSite

How can I use yum to download source packages instead of manually downloading every single package from RHN / CentOS / RedHat Ftp website?

You need to install a package called yum-utils. yum-utils is a collection of utilities and examples for the yum package manager. It includes utilities by different authors that make yum easier and more powerful to use. These tools include: debuginfo-install, package-cleanup, repoclosure, repodiff, repo-graph, repomanage, repoquery, repo-rss, reposync, repotrack, verifytree, yum-builddep, yum-complete-transaction, yumdownloader, yum-debug-dump and yum-groups-manager. Type the following to install yum-utils:
# yum install yum-utils
Once installed, create a repo for SRPM as follows:
# vi /etc/yum.repos.d/srpm.repo
Append the following code:

[rhel-src]
name=Red Hat Enterprise Linux $releasever - $basearch - Source
baseurl=ftp://ftp.redhat.com/pub/redhat/linux/enterprise/$releasever/en/os/SRPMS/
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release

Save and close the file.

How do I download php srpm file?

Simply use the command as follows:
# yumdownloader --source {packagename}
# yumdownloader --source php
Sample output:

Loaded plugins: rhnplugin
rhel-src                                                                                                                               |  951 B     00:00
primary.xml.gz                                                                                                                         | 615 kB     00:02
rhel-src                                                       2266/2266
Enabling epel-source repository
epel-source                                                                                                                            | 1.9 kB     00:00
primary.sqlite.bz2                                                                                                                     | 662 kB     00:00
Enabling epel-source repository
php-5.1.6-23.el5.src.rpm                                                                                                               | 8.0 MB     00:20

To list file, enter:
# ls *src.rpm