创建存储在字符串中的链接表单主题标签

For example I have this variable:

$test = "#stack #overflow #facebook";

Now I want to make simple link with this hashtags such as:

<a href="http:/facebook.com/stack">stack</a>
<a href="http:/facebook.com/overflow">overflow</a>
<a href="http:/facebook.com/facebook">facebook</a>

I don't know which hastags stored this variable and how can I do this action simply by `str_replace?

I read this link but I can do that:

https://stackoverflow.com/a/5862395/1830228

Converting my comment to answer.

You can use a preg_replace like this to support UTF-8:

$test = preg_replace('/#([\pL\pN_-]+)/u',
          '<a href="http:/facebook.com/$1">$1</a>', $test);

Regex #[\pL\pN_-]+ matches a # followed by 1+ unicode alphanumeric or _ or - characters.