无法使用composer在symfony2中安装mongodb doctrine

我按照官方文档

当我运行composer update时出现此错误:您的要求无法解析为可安装的软件包集.

  Problem 1
    - doctrine/mongodb 1.0.4 requires ext-mongo >=1.2.12,<1.6-dev -> the requested PHP extension mongo is missing from your system.
    - doctrine/mongodb 1.0.3 requires ext-mongo >=1.2.12,<1.5-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 requires ext-mongo >=1.2.12,<1.4-dev -> the requested PHP extension mongo is missing from your system.
    - doctrine/mongodb-odm 1.0.0-BETA9 requires doctrine/mongodb 1.0.* -> satisfiable by doctrine/mongodb[1.0.0, 1.0.1, 1.0.2, 1.0.3, 1.0.4].
    - Installation request for doctrine/mongodb-odm 1.0.0-BETA9 -> satisfiable by doctrine/mongodb-odm[1.0.0-BETA9].

当我运行时,php -m我找不到mongo扩展,但服务器正在运行,我可以在PHP中使用Mongo,有谁知道应该是什么问题？我怀疑这个学说找不到mongo扩展..谢谢!