I’ve encountered following error when running mongodb php module

Dec 12 09:56:02 lj php-fpm[3142]: [12-Dec-2016 09:56:02] NOTICE: PHP message: PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/lib64/php/modules/mongodb.so' - /usr/lib64/php/modules/mongodb.so: undefined symbol: php_json_serializable_ce in Unknown on line 0

Few google searches reveal this can be fixed by simply making sure json is loaded prior to mongodb.so – which by default isn’t.
There are many ways to do that, for example find how your json is loaded

[root@lj php.d]# php -i | grep "json"
/etc/php.d/json.ini,

Now open json.ini and add

extension=mongodb.so

at the end – and remove mongodb.ini from the folder.

Or rename json.ini to 20-json.ini and mongodb.ini to 30-mongodb.ini
You can also add priority at top of the ini file priority=30

Don’t forget to restart php after each change, if you are running mod_php, just do systemctl restart httpd, for php-fpm it’s systemctl restart php-fpm.

Now the weird thing is – none of this worked for me.
No matter how I loaded mongodb, it always failed with this error. I got the latest pecl packages, everything up to date – it simply didn’t work.
The only solution for me was to roll back to previous version of mongodb

[root@lj php.d]# pecl uninstall mongodb
[root@lj php.d]# pecl install mongodb-1.1.9
[root@lj php.d]# echo "extension=mongodb.so" > /etc/php.d/zz-99-mongo.ini

I called the file zz-99 to make sure it’s always loaded last. My ini files do not start with 20-*.ini to set priority.
Good luck.