php - How can I get last segment of the string?

Summery:

I need to get last part of this string:

$str = 'http://localhost:8000/news/786425/fa';
//                              last part ^^
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How can I do that?

Explanation:

I'm trying to either add a language in the end of URL (if it isn't exist) or replace it withen (if a language already exists). My website supports only two languages:en,fa. So justen andfa should be detected as languages. In other word, they are allowed languages, anything else would considered as a URL's parameter.

Examples:

$str = 'http://localhost:8000/news/786425/fa';
// expected output: http://localhost:8000/news/786425/en

 $str = 'http://localhost:8000/news/786425';
// expected output: http://localhost:8000/news/786425/en

 $str = 'http://localhost:8000/news/786425/pg';  // pg is not defined as a language
// expected output: http://localhost:8000/news/786425/pg/en


 $str = 'http://localhost:8000/news/786425/en';
// expected output: http://localhost:8000/news/786425/en
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Here is what I've tried so far.