jawaban

MunculSekali.php

<?php

function munculSeklali($string)
{
	$arr = str_split($string);
	foreach ($arr as $key => $val) {
		$tmp[$val] = isset($tmp[$val]) ? false : true;
	}

	$out = array_filter($tmp);
	var_dump(array_keys($out));
}

PesanRahasia.php

<?php

function pesan_rahasia($message)
{
	$messageLength = strlen($message);
	$K = ceil(sqrt($messageLength));
	$squareLength = pow($K, 2);
	$newMessage = str_pad($message, $squareLength, '*');
	$arrMessage = str_split($newMessage);
	$chunk = array_chunk($arrMessage, $K);
	$trans = array_map(null, ...$chunk);

	foreach ($trans as $key => $value) {
		$out[$key] = array_reverse($value);
		$out[$key] = implode('', $out[$key]);
	}

	$out = implode('', $out);
	$out = str_replace('*', '', $out);

	echo $out . PHP_EOL;
}