Description of problem:
- This is a RPM dependency problem.
- ganglia-web requires php-xml but ganglia-web-3.7.5-33.el8.x86_64.rpm's "requires" section does not include it.

- Without php-xml,
  we get errors like below in webpage(http://serverIP/ganglia)
This page isn’t working 
10.181.113.150 is currently unable to handle this request.
HTTP ERROR 500
  and  we get errors like below in /var/log/php-fpm/www-error.log 
# tail -n 6 /var/log/php-fpm/www-error.log 
[21-Oct-2021 02:33:37 UTC] PHP Fatal error:  Uncaught Error: Call to undefined function xml_parser_create() in /usr/share/ganglia/ganglia.php:356
Stack trace:
#0 /usr/share/ganglia/get_ganglia.php(11): Gmetad('127.0.0.1', 8652)
#1 /usr/share/ganglia/index.php(7): include_once('/usr/share/gang...')
#2 {main}
  thrown in /usr/share/ganglia/ganglia.php on line 356

Version-Release number of selected component (if applicable):
ganglia-web-3.7.5-33.el8.x86_64.rpm

How reproducible:

Steps to Reproduce:
1. OS installation
install OS(RHEL8.4) from DVD
base enviromemnt is "Server" without any addtional package.

2  ganglia installation
dnf -y install php php-gd rrdtool apr libmemcached-libs
rpm -ivh ganglia-web-3.7.5-33.el8.x86_64.rpm ganglia-gmetad-3.7.2-33.el8.x86_64.rpm ganglia-3.7.2-33.el8.x86_64.rpm libconfuse-3.3-1.el8.x86_64.rpm ganglia-gmond-3.7.2-33.el8.x86_64.rpm

To use dnf easiry, we did like 
  https://access.redhat.com/solutions/3776721

3. settings
3.1  igonore firewall/selinux
firewall-cmd --zone=trusted  --change-interface=ens3 --permanent
setenforce 0


3.2   Edit /etc/httpd/conf.d/ganglia.conf to avoid forbidden problem
# diff /etc/httpd/conf.d/ganglia.conf{,.orig}
9c9
< #  Deny from all
---
>   Deny from all
13,14d12
<   Allow from all
<  Require all granted

3.3  ower change from ganglia to apache
chown -R apache:apache /usr/share/ganglia

3.4 start gmond/gmetad and restart httpd
service httpd start
service gmond start
service gmetad start
service httpd restart

Then we see
# tail -f /var/log/php-fpm/www-error.log 
[21-Oct-2021 08:12:43 UTC] PHP Fatal error:  Uncaught Error: Call to undefined function xml_parser_create() in /usr/share/ganglia/ganglia.php:356
Stack trace:
#0 /usr/share/ganglia/get_ganglia.php(11): Gmetad('127.0.0.1', 8652)
#1 /usr/share/ganglia/index.php(7): include_once('/usr/share/gang...')
#2 {main}
  thrown in /usr/share/ganglia/ganglia.php on line 356

Actual results:
# rpm -qp -R ganglia-web-3.7.5-33.el8.x86_64.rpm 
warning: ganglia-web-3.7.5-33.el8.x86_64.rpm: Header V4 RSA/SHA256 Signature, key ID 2f86d6a1: NOKEY
/bin/bash
/bin/sh
config(ganglia-web) = 3.7.5-33.el8
ganglia-gmetad = 3.7.2-33.el8
php
php-gd
rpmlib(BuiltinLuaScripts) <= 4.2.2-1
rpmlib(CompressedFileNames) <= 3.0.4-1
rpmlib(FileDigests) <= 4.6.0-1
rpmlib(PayloadFilesHavePrefix) <= 4.0-1
rpmlib(PayloadIsXz) <= 5.2-1
rrdtool


Expected results:
# rpm -qp -R ganglia-web-3.7.5-33.el8.x86_64.rpm 
warning: ganglia-web-3.7.5-33.el8.x86_64.rpm: Header V4 RSA/SHA256 Signature, key ID 2f86d6a1: NOKEY
/bin/bash
/bin/sh
config(ganglia-web) = 3.7.5-33.el8
ganglia-gmetad = 3.7.2-33.el8
php
php-gd
php-xml
rpmlib(BuiltinLuaScripts) <= 4.2.2-1
rpmlib(CompressedFileNames) <= 3.0.4-1
rpmlib(FileDigests) <= 4.6.0-1
rpmlib(PayloadFilesHavePrefix) <= 4.0-1
rpmlib(PayloadIsXz) <= 5.2-1
rrdtool

Additional info:
even after installing php-xml,
we get 
There was an error collecting ganglia data (127.0.0.1:8652): fsockopen error: Permission denied
but this is another problem
, which is fixed with "setenforce 0"