laravel5安装问题

composer create-project laravel/laravel --prefer-dist 执行这条语句

ppledeMBP:laravel5 apple$ composer create-project laravel/laravel --prefer-dist
Installing laravel/laravel (v5.0.1)
- Installing laravel/laravel (v5.0.1)
Loading from cache

Created project in /Users/apple/www/laravel5/laravel
Loading composer repositories with package information
Installing dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

Problem 1
- laravel/framework v5.0.2 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- laravel/framework v5.0.1 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- laravel/framework v5.0.0 requires ext-mcrypt * -> the requested PHP extension mcrypt is missing from your system.
- Installation request for laravel/framework 5.0.* -> satisfiable by laravel/framework[v5.0.0, v5.0.1, v5.0.2].

不知道什么问题