example.php

<?php
require_once(__DIR__.'/functions.php')

stream_wrapper_register("proc", "ProcStream");

$user = opt('u', ['svnuser']);

$reader = new XMLReader();
$reader->open('proc://svn log -l 3 --xml');

// fast forward to first logentry
while ($reader->read() && $reader->name !== 'logentry');

while ($reader->name === 'logentry') {
    $node = new SimpleXMLElement($reader->readOuterXML());
    printf("%s: %s", $node->author, $node->msg);
    $reader->next('logentry');
}

ProcStream.php

<?php

class ProcStream {

    /**
     * @var resource
     */
    protected $process;

    /**
     * @var array
     */
    protected $pipes;

    public function stream_open($path, $mode, $options, &$opened_path) {
        $url = parse_url($path);
        $start = strlen($url['scheme']) + 3; // 3 = strlen('://')
        $command = substr($path, $start);

        $descriptorSpec = array(
            0 => array("pipe", "r"), // stdin is a pipe that the child will read from
            1 => array("pipe", "w"), // stdout is a pipe that the child will write to
            2 => array("pipe", "w") // stderr is a pipe that the child will write to
        );

        $this->process = proc_open($command, $descriptorSpec, $this->pipes, realpath('./'));
        if (!is_resource($this->process)) {
            return false;
        }

        return true;
    }

    public function stream_read($count) {
        return fread($this->pipes[1], $count);
    }

    public function stream_write($data) {
        return fwrite($this->pipes[0], $data);
    }

    public function stream_eof() {
        $status = proc_get_status($this->process);
        return !$status['running'];
    }

    public function stream_close() {
        return proc_close($this->process);
    }

    public function url_stat($path, $flags) {
        return [];
    }
}