For php7.0 in ubuntu 14.04

install_pcel_mongodb.sh

#!/bin/bash

sudo apt-get install php-pear
sudo pecl channel-update pecl.php.net
sudo apt-get install -y php7.0-dev
sudo apt-get install pkg-config libssl-dev
sudo pecl install mongodb

// add extension=mongodb.so to /etc/php/7.0/fpm/php.ini

//add extension=mongodb.so in fpm and cli:
//sudo nano /etc/php/7.0/fpm/conf.d/30-mongodb.ini
//sudo nano /etc/php/7.0/cli/conf.d/30-mongodb.ini

sudo restart php7.0-fpm
sudo service nginx restart