将相对路径转换为绝对路径的最佳方法是什么？

I am making a search script and want to stop any deep level search for files and folders outside of the web document root.

In case the user passes in something like ../../../../../../ I want to capture it and convert it into the absolute path that it points to, then perform some sanitation checks.

So in the example if ../../../../../ is converted to / I want to replace / with $_SERVER['DOCUMENT_ROOT'] or any other function in PHP to push the search and lock it in the web server root.

You can use the function "realpath":

echo realpath('../../../../../../');