<?php

function hex2char($matches){
    return chr(hexdec('0x' . $matches[1]));
}

class II_Xml_Filter extends php_user_filter {
    
    private $buffer;
    private $boundary = "\n"; // boundary string
            
    /**
     * This method is called on fwrite, fread, etc
     *
     * @param bucket_brigade $in
     * @param bucket_brigade $out
     * @param integer $consumed
     * @param boolean $closing
     * @return integer PSFS_* constants
     */
    function filter($in, $out, &$consumed, $closing) 
    {     
        /**
         * We must create the bucket buffer here because contructor is never called 
         * (this gets instantiated internally within PHP and constructor is not automatically called)
         * 
         * onCreate() can't be used either, because $this->stream is not available then
         */
        if(!$this->buffer){
            $this->buffer = stream_bucket_new($this->stream, '');
        }
               
        while ($bucket = stream_bucket_make_writeable($in)) {
            
            // the last "safe" position for replacement
            $lastBoundary = strripos($bucket->data, $this->boundary);
            
            if($lastBoundary === false){
                // boundary not found, buffer the bucket and FEED ME more data!
                $this->buffer->data .= $bucket->data;
                return PSFS_FEED_ME;
            }else{
                $lastBoundary += strlen($this->boundary);
            }
            
            // buffer + chars from bucket up until (and including) the last boundary string
            $toReplace = $this->buffer->data 
                       . substr($bucket->data, 0, $lastBoundary);
            
            // characters from the last boundary string to the end of the bucket data
            $this->buffer->data = substr($bucket->data, $lastBoundary);
            
            // do string replace
            $bucket->data = $this->replace($toReplace);
            
            // ?? no idea if this is correct :-/
            $consumed += strlen($toReplace);
            
            // stick it into the out brigade
            stream_bucket_append($out, $bucket);
        }
        
        // this is the last time through, so flush the buffer onto the output brigade
        if($closing && $this->buffer->data){
            $this->buffer->data = $this->replace($this->buffer->data);
            stream_bucket_append($out, $this->buffer);
        }
        
        // pass on to the next filter
        return PSFS_PASS_ON;
    }
    
    function replace($str)
    {
        return preg_replace_callback('/_x([a-f0-9]{4})_/iu', 'hex2char', $str);
    }
}


// usage:-

$filename = 'output.txt';

stream_filter_register("II_Xml_Filter", "II_Xml_Filter");

$fp = fopen($filename, "w");

if($fp){
    stream_filter_append($fp, "II_Xml_Filter");
    
    // cross boundary test
    fwrite($fp, "o_x0021_o_x0");
    fwrite($fp, "021_o\n");
    fclose($fp);
    
    readfile($filename);
}else{
    echo 'could not open ', $filename;
}