undefined symbol: php_json_encode in Unknown on line 0.

The issue is with the loading order, so the json extension needs to be loaded before mongo.so is loaded.
Since others are likely to come across this I will outline the whole process:

In your /etc/php/mods-available directory (or as appropriate to platform) create a separate mongo.ini with the following:

; configuration for php mongo module; priority=30extension=mongo.so

Remove any other references to mongo.so from other files such as php.ini

Create symlinks in each of the cli and apache2 directories as required for use as so:

sudo ln -s ../../mods-available/mongo.ini 30-mongo.ini
At end of this you should have a structure that looks like this

$/etc/php5$ tree.├── apache2│├── conf.d││├──05-opcache.ini ->../../mods-available/opcache.ini││├──10-pdo.ini ->../../mods-available/pdo.ini││├──20-json.ini ->../../mods-available/json.ini││├──20-readline.ini ->../../mods-available/readline.ini││└──30-mongo.ini ->../../mods-available/mongo.ini│└── php.ini├── cli│├── conf.d││├──05-opcache.ini ->../../mods-available/opcache.ini││├──10-pdo.ini ->../../mods-available/pdo.ini││├──20-json.ini ->../../mods-available/json.ini││├──20-readline.ini ->../../mods-available/readline.ini││└──30-mongo.ini ->../../mods-available/mongo.ini│└── php.ini└── mods-available    ├── json.ini    ├── mongo.ini    ├── opcache.ini    ├── pdo.ini    └── readline.ini

This makes sure that the "json"extension will be loaded by the dynamic loader before the "mongo"module is.
But basically remove the mongo.so from "php.ini"and put it in it's own file with higher loading order than the json extension. Then it will work.
This possibly needs a JIRA as I believe it has come up before.
Come from:http://stackoverflow.com/questions/22952994/mongo-so-undefined-symbol-php-json-encode-in-unknown-on-line-0-after-instal