This can be caused by a faulty plug-in.
1. Go back to a php version that the website works.
2. Then in WordPress de-activate all the plugins via Dashboard->Plugins menu button on the left.
3. In case you are unable to reach WordPress dashboard.
You can also manually de-activate the plugins via an FTP editor, like FileZilla software. Go to your server folder where WordPress is installed , then
in the /wp-content/ the plugins folder is present.
How to: De-activating all the WordPress plugins manually:
In /wp-content/ folder temporarily renaming the plugins folder to for example plugins-off disables all the WordPress plugins. Later you can remame it back to its original name. Then activate them one by one in the WordPress dashboard, as described in step 5.
4. Try updating to the desired php now and see if the problem is solved.
5. To find which plugin is causing the problem, activate the plugins one by one.
If you have renamed the plugins folder as explained in step 3, remember to rename it to the original plugins name first, before activating the plugins in WordPress dashboard.
One of the plugins that we had which often cause this issue is is Slider Revolution plugin.