Fatal error: Call to undefined function simplexml_load_string() in ubuntu 16 or ubuntu 18

Fatal error: Call to undefined function simplexml_load_string()

execute below command:  

$ sudo apt-get install python-software-properties
$ sudo add-apt-repository ppa:ondrej/php
$ sudo apt-get update
$ sudo apt-get install -y php5.6

then:

sudo apt-get install php5.6-mbstring php5.6-mcrypt php5.6-mysql php5.6-xml php5.6-url

and finally restart server:

sudo service apache2 restart