gistfile1.php

<?php
class foo
{
    function stream_open() { return TRUE; }
    function stream_eof() { return FALSE; }

    function stream_read($count)
    {
        $backtrace = debug_backtrace();
        if (isset($backtrace[1]['function']) && $backtrace[1]['function'] === 'fread') {
            $count = $backtrace[1]['args'][1];
        }

        var_dump($count);
        return "";
    }
}

stream_wrapper_register('foo', 'foo');

$handle = fopen('foo://bar', NULL);
fread($handle, 1);     // int(1)
fread($handle, 16384); // int(16384)