解決php5-fpm ( nginx ) "Call to undefined function json_encode" 錯誤

系統：
ubuntu 14.04

用nginx跑php時找不到json_encode()函數報錯：
Call to undefined function json_encode ... 

但使用apache2跑則正常

檢查：
# dpkg --get-selections | grep php5-json
有安裝php5-json

# apt-get install php5-json
Reading package lists... Done
Building dependency tree
Reading state information... Done
php5-json is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
已安裝，所以沒新東西被裝

但是
# php5-fpm -m | grep json
(無結果)

root@ubuntu:/etc/php5/fpm/conf.d# ls -l
total 0
lrwxrwxrwx 1 root root 32 Jul  3 11:11 05-opcache.ini -> ../../mods-available/opcache.ini
lrwxrwxrwx 1 root root 28 Jul  3 11:11 10-pdo.ini -> ../../mods-available/pdo.ini
lrwxrwxrwx 1 root root 29 Jul  3 11:11 20-curl.ini -> ../../mods-available/curl.ini
lrwxrwxrwx 1 root root 27 Jul  3 11:11 20-gd.ini -> ../../mods-available/gd.ini
lrwxrwxrwx 1 root root 31 Jul  3 11:11 20-mcrypt.ini -> ../../mods-available/mcrypt.ini
lrwxrwxrwx 1 root root 31 Jul  3 11:11 20-mysqli.ini -> ../../mods-available/mysqli.ini
lrwxrwxrwx 1 root root 30 Jul  3 11:11 20-mysql.ini -> ../../mods-available/mysql.ini
lrwxrwxrwx 1 root root 34 Jul  3 11:11 20-pdo_mysql.ini -> ../../mods-available/pdo_mysql.ini
lrwxrwxrwx 1 root root 33 Jul  3 11:11 20-readline.ini -> ../../mods-available/readline.ini
原來json.ini不在php5-fpm的設定裡面

把php5-fpm的conf.d備份，然後重建
root@ubuntu:/etc/php5/fpm# mv conf.d/ conf-old.d
root@ubuntu:/etc/php5/fpm# ln -s /etc/php5/mods-available /etc/php5/fpm/conf.d

然後重啟
# service nginx restart
# service php5-fpm restart  // 注意：重啟nginx是無效的，要重啟動php5-fpm

檢查：
# php5-fpm -m | grep json
json

最後打開網頁，json_encode()可以正常使用