Code

gistfile1.php

<?php
echo "output starts here".PHP_EOL;

$php = new Runkit_Sandbox();
$php['output_handler'] = 'test_handler';
$php->echo("foo\n");
$php->echo("Barish\n");
$php->echo("BAZimbly\n");
var_dump($php['output_handler']);

function test_handler($str) {
  if (strlen($str) == 0) return NULL; /* Do nothing with flush events */
  /* Echoing and returning have the same effect here, both go to parent's output chain */
  echo 'Received string from sandbox: ' . strlen($str) . " bytes long.\n";

  return strtoupper($str);
}

echo "end output".PHP_EOL;

Output

output starts here
string(12) "test_handler"
end output
Received string from sandbox: 20 bytes long.
FOO
BARISH
BAZIMBLY