Zend_extentions are extensions that are built into the Zend engine itself. The engine parses, interprets and executes PHP script. Changing the engine changes the way that PHP works.
The included diagram will describe the main difference between an extension and a zend_extension.
Both types of extensions share lots of functionality. The difference between both types is mainly with the hooks that they register into the Engine. Despite it being very uncommon, an extension can be both a PHP extension and a Zend extension. Xdebug is a good example.
Find below image for main schema
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