<?php
class Human extends Animal
{
}

class Animal
{
    protected $skin;

    function __construct()
    {
        $this->skin = rand(1, 10000);
    }

    function getSkin()
    {
        return $this->skin;
    }
}

function make_random_array($count, $class)
{
    $arr = [];
    for ($i = 0; $i < $count; $i++) {
        $arr[] = new $class();
    }
    return $arr;
}

$humans = make_random_array(300000, 'Human');

$ignored = make_random_array(2000, 'Animal');

$start = microtime(true);

$ignored_skins = [];
foreach ($ignored as $skin){
    $ignored_skins[$skin->getSkin()] = true;
}

$suitable_humans = [];
foreach($humans as $humen){
    if(!isset($ignored_skins[$humen->getSkin()])){
        $suitable_humans[] = $humen;
    }
}

$end = microtime(true);

echo 'done for ' . ($end - $start) . ' ms' . PHP_EOL;
echo 'out of ' . count($humans) . ' humans ' . count($suitable_humans) . ' are have suitable skin' . PHP_EOL;