It appears that many people or sys admin want to run php 5 on Red Hat Enterprise Linux. Unfortunately Redhat does not provide this package (RPM file) for RHEL. You can download source code and install php 5 from official php site. This requires compiler collection on your system. Download source code and just follow instructions presented in INSTALL or REDME file.
Another option is search and installs PHP 5 packages. You can download x86_64 PHP 5 package here. Use wget command to download these packages and rpm command to install new packages. But first remove old php4 package using rpm -e command.
Alert: This post is outdated. Please use the latest version of RHEL/CentOS v6.x+ for PHP 5.x. The author no longer support php 5 rpm on RHEL 4.x.
However these rpm packages 64 bit so if you are running 32 bit os rebuild RPM file.
Step # 1: Download src rpm
# cd /tmp
# wget http://www.cyberciti.biz/files/lighttpd/rhel4-php5-fastcgi/php-5.1.4-1.esp1.src.rpm
# rpm -ivh php-5.1.4-1.esp1.src.rpm
Step #2: Rebuild RPM for 32 bit RHEL version
# cd /usr/src/redhat/SPECS
# rpmbuild -bb php.spec
It will take some time to compile and rebuild RPM files.
Step #3: Install new php5 RPM file
Remove old php4 rpm. Go to /usr/src/redhat/RPMS directory and install PHP5 RPM files.
Please note that PHP 5 is not official supported by Red Hat on RHEL 4.0. You are using these packages on your own risk ;)