在ubuntu 18.04中，用php5.6，php5.6-fpm，nginx，用ubuntu的mongodb

composer require mongodb/mongodb

错误

Using version ^1.4 for mongodb/mongodb
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

Problem 1
    - mongodb/mongodb 1.4.2 requires ext-mongodb ^1.5.0 -> the requested PHP extension mongodb has the wrong version (1.4.2) installed.
    - mongodb/mongodb 1.4.1 requires ext-mongodb ^1.5.0 -> the requested PHP extension mongodb has the wrong version (1.4.2) installed.
    - mongodb/mongodb 1.4.0 requires ext-mongodb ^1.5.0 -> the requested PHP extension mongodb has the wrong version (1.4.2) installed.
    - Installation request for mongodb/mongodb ^1.4 -> satisfiable by mongodb/mongodb[1.4.0, 1.4.1, 1.4.2].