A PHP update by your web host is expected to go smoothly, but sometimes these updates can make things worst. This is because different webhosts have different configurations of their severs. This is also why you need to put your site in the best hosting server you can afford so you won’t face problems when upgrading a software like PHP. Choosing an unreliable web host can cause unexpected downtime on or during an update. You can’t do anything about it and worse, you’ll end up switching to a different host.
When your web host updates PHP, expect that things will go smoothly but have a fallback plan of your own.
Backup your wordpress site. Either do a manual backup or subscribe to any backup solutions provider like VaultPress and BackWPup.
After creating your backup be sure that you are running the latest stable version of WordPress. Also, make sure your plugins and themes are up-to-date.
Although it is unlikely, but if PHP update breaks your site, the first thing you need to do is to determine the root cause of the problem. Deactive all your plugins, then activate it one by one to see what works and what doesn’t.
If it does not solve the issue, read the rest of the post here: http://www.wpbeginner.com/beginners-guide/how-does-php-updates-by-your-web-host-impacts-your-wordpress-sites/