Errors installing Composer

I'm trying to install Composer by following the directions in the Command-line installation. When I run
php composer-setup.php
I get these errors:

Some settings on your machine make Composer unable to work properly.
Make sure that you fix the issues listed below and run this script again:

The filter extension is missing.
Install it or recompile php without --disable-filter

The iconv OR mbstring extension is required and both are missing.
Install either of them or recompile php without --disable-iconv

If you could, please give me explicit direction on how to fix it. Thanks for your time.