Just on the Joomla 3.8.8 "You have post-installation messages" page and when I click the "Read Messages" button I get taken to a page with an error message:

At the moment I have plenty of XML modules installed but not the one I need:

php -m | grep -i xml
libxml
SimpleXML
xml

I need to have the xmlreader module installed which apparently is in the php-xml phpXX-xml package

yum install php71-xml
php -m |grep -i xml
libxml
SimpleXML
xml
xmlreader
xmlwriter

To check if you have a class installed in PHP you can run this snippet:

 <?php


$exists = "XMLReader"; # the class you are looking for

# does it exist?
if(class_exists($exists)){
	echo "Class Exists! " . $exists . "\n";
} else {
	echo "Class does not exist! " . $exists . "\n";
}

# if it's in a file somewhere and not in the default binary then
# reflectionClass will tell you the location
# or return false
$reflect = new ReflectionClass($exists);

echo var_dump($reflect->getFileName()) . "\n";