PHP return_printed function that takes a closure and returns as a string anything that was printed by it. (Thanks to Viper-7 in ##php, see his version that supports arguments http://codepad.viper-7.com/7ZSncY )

return_printed.php

function return_printed() {
  $args = func_get_args();
  $f = array_shift($args);
  if ($f) {
    ob_start();
    call_user_func_array($f, $args);
    return ob_get_clean();
  }
}

// return_printed(function($s) { print "hello world" . $s; }, " - argument");
// - "hello world"