服务器上安装composer总是失败，貌似是验证失败，不知道该怎么解决。我是按照官方文档操作的，过程如下：

php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
php composer-setup.php

执行到这里之后就不行了，报如下错误

Downloading...
Signature mismatch, could not verify the phar file integrity
Retrying...

过一会之后错误信息变成下面这样

Downloading...
Signature mismatch, could not verify the phar file integrity
Retrying...
The "https://getcomposer.org/download/1.4.1/composer.phar" file could not be downloaded: >failed to open stream: HTTP request failed!
Retrying...
Signature mismatch, could not verify the phar file integrity
The download failed repeatedly, aborting.

然后就停止了。好无奈，试了很多遍都一样。
我直接下载执行文件（composer.phar）倒是可以执行，但是好像就不能全局安装了，所以还是想按照文档上的方式执行成功，不知道我该如何解决！