Filter the numbers that have no pair (Yandex interview)

LonelyNumberFilter.php

<?php

class LonelyNumberFilter {

	private $numbers;
	private $filtered = [];
	
	function __construct($numbers) {
		$this->numbers = $numbers;
	}
	
	public function apply() {
		foreach ($this->numbers as $i => $number) {
			if ($this->haveSeen($number)) {
				$this->markAsPaired($number, $i);
			}
			else {
				$this->markAsLonely($number, $i);
			}
		}
	
		return $this->getLonelyNumbers();
	}
	
	private function haveSeen($number) {
		return $this->alreadyBeenMarkedAsLonely($number);
	}
	
	private function alreadyBeenMarkedAsLonely($number) {
		return isset($this->filtered[$number]);
	}
	
	private function markAsPaired($number, $i) {
		$this->unmarkAsLonely($number);
	}
	
	private function unmarkAsLonely($number) {
		unset($this->filtered[$number]);
	}
	
	private function markAsLonely($number, $i) {
		$this->filtered[$number] = $i;
	}
	
	private function getLonelyNumbers() {
		return $this->filtered;
	}

}

$data = array_fill(0, 3000000, 1);
$data[] = 2;

$time_start = microtime(true);
$theNumber = (new LonelyNumberFilter($data))->apply();
$time_end = microtime(true);

$time = $time_end - $time_start;

print_r($theNumber);
echo "$time ms";

Output:
Array ( [2] => 3000000 ) 2.0812890529633 ms