If you use 'php -m', it list mongodb. That you have a right mongdb extension version for your PHP. Otherwise, you have a wrong mongoDB extension version. If you have a right mongodb extension, but still meet the error. I think your server software (ex: apache) have a permission problem. You can try with Administrator right for it. Run As Adminstrator with windows or sudo when use *nix. Apache run PHP with php path, I set PHP path on environment path. It's solved my issue.

If you use 'php -m', it list mongodb. That you have a right mongdb extension version for your PHP. Otherwise, you have a wrong mongoDB extension version. If you have a right mongodb extension, but still meet the error. I think your server software (ex: apache) have a permission problem. You can try with Administrator right for it. Run As Adminstrator with windows or sudo when use *nix.