<?php

error_reporting(-1);
mb_internal_encoding('utf-8');

$regexp = array(
	"en" => '/[bDdFfGghIiJjkLlmNnQqRrSstUuVvWwYZz]/',
);

$lettersRU = array(
	'A' => 'А',
	'a' => 'а',
	'B' => 'В',
	'C' => 'С',
	'c' => 'с',
	'E' => 'Е',
	'e' => 'е',
	'H' => 'Н',
	'K' => 'К',
	'M' => 'М',
	'O' => 'О',
	'o' => 'о',
	'P' => 'Р',
	'p' => 'р',
	'T' => 'Т',
	'X' => 'Х',
	'x' => 'х',
	'y' => 'у'
);
$lettersEN = array_flip($lettersRU);

$text = "Пpивeт всeм! Tоday мы погoворим о тaкoй штyке, кaк базовые пoтребноcти.";
$words = explode(" ", $text);

foreach($words as $ind => &$word) {
	if(preg_match($regexp["en"], $word)) {
		$word = strtr($word, $lettersEN);
	} else {
		$word = strtr($word, $lettersRU);
	}
}

$correctText = implode(" ", $words);

echo $correctText;