I ran: composer require mongodb/mongodb. But it gave me the following error :

• mongodb/mongodb 1.0.1 requires ext-mongodb ^1.1.0 -> the requested PHP extension mongodb is missing from your system.
• mongodb/mongodb 1.0.0 requires ext-mongodb ^1.1.0 -> the requested PHP extension mongodb is missing from your system.
• Installation request for mongodb/mongodb ^1.0 -> satisfiable by mongodb/mongodb[1.0.0, 1.0.1].

I tried : composer show -p

Then it gave me:
......
ext-iconv 0 The iconv PHP extension
ext-mcrypt 0 The mcrypt PHP extension
ext-mhash 0 The mhash PHP extension
ext-mongo 1.6.12 The mongo PHP extension
ext-mysql 1.0 The mysql PHP extension
ext-mysqli 0.1 The mysqli PHP extension
......

Why am I getting the error?

php -m

.....
libxml
mbstring
mcrypt
mhash
mongo
mysql
mysqli
openssl
pcntl
pcre
PDO
.....

I'm using ubuntu 14.04 LTS