不能使用Composer安装doctrine/mongodb-odm

试图通过Composer安装doctrine/mongodb-odm我得到这个:

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
    - doctrine/mongodb 1.0.x-dev requires ext-mongo >=1.2.12,<1.4-dev -> the requested PHP extension mongo is missing from your system.
    - doctrine/mongodb 1.0.2 requires ext-mongo >=1.2.12,<1.4-dev -> the requested PHP extension mongo is missing from your system.
    - doctrine/mongodb 1.0.1 requires ext-mongo >=1.2.12,<1.4-dev -> the requested PHP extension mongo is missing from your system.
    - doctrine/mongodb 1.0.0-BETA3 requires ext-mongo >=1.3.1,<1.4-dev -> the requested PHP extension mongo is missing from your system.
    - doctrine/mongodb 1.0.0-BETA2 requires ext-mongo >=1.2.12,<1.3-dev -> the requested PHP extension mongo is missing from your system.
    - doctrine/mongodb 1.0.0-BETA1 requires ext-mongo * -> the requested PHP extension mongo is missing from your system.
    - doctrine/mongodb 1.0.0 requires ext-mongo >=1.2.12,<1.4-dev -> the requested PHP extension mongo is missing from your system.
    - Installation request for doctrine/mongodb 1.0.*@dev -> satisfiable by doctrine/mongodb 1.0.0, doctrine/mongodb 1.0.0-BETA1, doctrine/mongodb 1.0.0-BETA2, doctrine/mongodb 1.0.0-BETA3, doctrine/mongodb 1.0.1, doctrine/mongodb 1.0.2, doctrine/mongodb 1.0.x-dev.

然而php -m有:

[PHP Modules]
...
mongo
...

， composer.phar show --platform有:

...
ext-mongo        1.3.5     The mongo PHP extension
...

任何想法?扩展肯定是正确安装的，Composer甚至列出了它。版本(>=1.2.12，<1.4-dev)也满足要求。