How to make Magento 1.9.x compatible with PHP 7.
PHP 7 represents significant improvements over earlier versions, and its improved speed is especially beneficial for sites running Magento.
Magento 2.X supports PHP 7 from the get-go while Magento 1.9.x , latest version at the time of writing is 126.96.36.199, does not support PHP 7 out of the box.
The fastest way to get magento from 1.9.2.X and up to work on PHP 7 is to use Inchoo’s PHP 7 compatibility extension for Magento 1.
Patching Magento 1.9.2 and up with the PHP 7 compatibility extension will make the core magento run fine on PHP 7.
If your shop has extensions added, these might still not be compatible with PHP 7.
You can contact the authors of your shops extensions and check for PHP 7 compatibility and if they do not support it then you can remove it, request them to make a new version that supports PHP 7 or as long as its not an encrypted extension modify it yourself to work on PHP 7.
A good tool to help in the process of checking and finding PHP 7 related issues the PHP 7 Compatibility Checker is a very useful tool.
This list of PHP 7’s backward incompatible changes is also useful if you want to modify the extensions yourself.