get full class name from short class name

Somewhere in the codebase there is a file abc.php with a class named abc:

<?php
namespace x\y\z;

class abc {
    // ...
}

In different script, a variable with short class name (without namespace):

$className = "abc";

I need a reflection of class, but the code:

  $r = new \ReflectionClass($className); //Error: Class abc does not exist in...

is not working since full class name "x\y\z\abc" is required.

Building an object also not working for the same reason:

$obj = new $className(); // Error: Class 'abc' not found in...

How to find the full class name from the short one ?

Any suggestions are welcome.