I guess you used this command to try to install it:
curl -sS https://getcomposer.org/installer | php
possibly your error is similar to this:
PHP Warning: file_get_contents(https://getcomposer.org/versions): failed to open stream: Connection timed out in - on line 762
PHP Warning: Invalid argument supplied for foreach() in - on line 508
None of the 0 stable version(s) of Composer matches your PHP version (5.4.45 / ID: 50445)
If this is your case, there are two possible solutions:
I think they could be proxy problems. Remove environment vars https_proxy and http_proxy should work for (Windows 10). And then, install them again, set the proxy blank. It should work
o I changed from https to http:
curl -sS http://getcomposer.org/installer%20| php