Oh also the bug disappears with:

    fseek($a, 4);
    stream_get_contents($a, 30);

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

This is because your stream_seek() function does not return the right value.
stream_seek() should return TRUE if the seek has successed, you return the value 
of fseek(), which is 0 on success.

Additionally, you have to implement stream_tell() so that the underlying layer 
can ask for it and know where the stream position is.
stream_tell() is called just after stream_seek() has successed, thus it is 
needed, even if you never call ftell() on your own stream, the underlying layer 
does.

Correct code is :

<?php

class StreamWrapper {

    protected $_stream = null;

    public function stream_open ( $path, $mode, $options, &$openedPath ) {

        $this->_stream = fopen(__FILE__, $mode);

        return true;
    }

    public function stream_seek ( $offset, $whence = SEEK_SET ) {

        var_dump('seek to ' . $offset);

        if (fseek($this->_stream, $offset, $whence) == 0) {
            return true;
        }
        return false;
    }

    public function stream_tell()
    {
        return ftell($this->_stream);
    }

    public function stream_read ( $count ) {

        return fread($this->_stream, $count);
    }

    public function stream_stat ( ) {

        return fstat($this->_stream);
    }

    public function stream_eof ( ) {

        return feof($this->_stream);
    }
}

stream_wrapper_register('foo', 'StreamWrapper');

$a = fopen('foo://bar', 'rb');
var_dump(stream_get_contents($a, 30, 4));

Oh yes, I forgot that tell() returns 0 on success… That's solved my issue. Thanks!