<?php

function comp_func($n1,$n2) {
  $n1=(int)$n1; $n2=(int)$n2;
  return $n1 === $n2 ? 0 : ($n1 > $n2 ? 1 : -1);
}

$a = array("0.1" => 9, "0.5" => 12, 0 => 23, 1 => 4, 2 => -15);
$b = array("0.2" => 9, "0.5" => 22, 0 => 3, 1 => 4, 2 => -15);
$result = array_udiff($a, $b, "comp_func");
var_dump($result);