I don't think this works correctly. It only checks for a full match of the namespace, but not for partial matches. Here is an example data provider for a unit test (which is also missing, btw):

    public function psr4Provider()
    {
        $classes   = [];
        $classes[] = [ 'L1\L2', 'src/Level/', 'L1\L2\L3\L4\File', 'src/Level/L3/L4/File.php' ];
        $classes[] = [ 'L1\L2', 'src/Level/', 'L1\L2\L4\File', 'src/Level/L4/File.php' ];
        $classes[] = [ 'L1\L2', 'src/Level/', 'L1\L2\File', 'src/Level/File.php' ];
        $classes[] = [ 'L1\L2', 'src/Level/', 'L1\File', 'L1/File.php' ];

        return $classes;
    }


based on this I came up with the following implementation:

    public function get_psr4_class_filepath($class)
    {
        $namespace = substr($class, 0, strrpos($class, '\\'));
        $classname = str_replace('\\', DIRECTORY_SEPARATOR, $class);
        $class     = substr($class, strrpos($class, '\\') + 1);

        do
        {
            if (isset($this->prefixes[$namespace]) === TRUE)
            {
                $base_path = $this->prefixes[$namespace];
                $class     = str_replace('\\', DIRECTORY_SEPARATOR, $class);

                return $base_path . $class . '.php';
            }
            else
            {
                $class     = substr($namespace, strrpos($namespace, '\\') + 1) . '\\' . $class;
                $namespace = substr($namespace, 0, strrpos($namespace, '\\'));
            }
        }
        while (strrpos($namespace, '\\') > 1);

        return $classname . '.php';
    }

we're not setting a namespace, we're setting the path for a namespace prefix, so IMHO "set_prefix" would be a better name.

adding an include path should never happen automagically. This can cause all kinds of really hard to debug problems.

I think the unique AutoloaderTest.php file deserves to be split now :/
I think we miss some testing with multiple prefixes set (to test the order mainly)
Let say for example we have as prefixes

- Foo\Bar     => /libs/yoyo

- Foo\Bar\Baz => /libs/hehe

and as files

- /libs/yoyo/Baz/Qux.php

- /libs/hehe/Qux.php

then I think

\Foo\Bar\Baz\Qux

should resolve to

the hehe libs file

I think psr4 should be looked at first

since this is the "official" one now
in case we have 2 files:

- libs/Foo/Bar_Baz.php

- libs/Foo/Bar/Baz.php
\Foo\Bar_Baz should resolve to the first file

while I think it will resolve to second one with the current code
This order thing should be unit tested as well then

I think we should have more than these

- use of underscore in namespace

- use of underscore in class name

- multiple levels of partial class

- multiple levels of partial path

-...

you never use $fullClass or $fullPath in here

You should use a different provider for this function if needed or none at all

I have a bit of trouble seeing what this test more than the previous function

Either remove or make the description more precise

Ship It!