PHP RegEx Replace accents of each selected word

Example :

$stopwords = ['toto', 'Pied', 'the'];
$str = 'bla bla toto bla bla Toto bla bla Tôtô bla bla totô bla bla bla bla the thé pied pïed pîéd';

$letters = [
  'a' => ['â', 'à'],
  'c' => ['ç'],
  'e' => ['é', 'ê', 'è', 'ë'],
  'i' => ['î', 'ï'],
  'o' => ['ô'],
  'u' => ['û', 'ù', 'ü'],
];
$s = ['â', 'à', 'ç', 'é', 'ê', 'è', 'ë', 'î', 'ï', 'ô', 'û', 'ù', 'ü'];
$r = ['a', 'a', 'c', 'e', 'e', 'e', 'e', 'i', 'i', 'o', 'u', 'u', 'u'];

// For each stop words ...
foreach ($stopwords as $stop_word) {
  $stop_word = str_replace($s, $r, strtolower($stop_word));
  $stopword_letters = str_split($stop_word);
  $reg = '/';
  // For each letter of each word ...
  foreach ($stopword_letters as $stopword_letter) {
    if (isset($letters[$stopword_letter])) {
      $reg .= '(';
      $reg .= $stopword_letter . '|';
      foreach ($letters[$stopword_letter] as $letter_replace) {
        $reg .= $letter_replace . '|';
      }
      $reg .= '\p{M})';
    }
    else {
      $reg .= $stopword_letter;
    }
  }
  $reg .= '/mi';
  echo "<br><b>$reg</b><br>";
// $re = '/t(?:ô|o|\p{M})t(?:ô|o|\p{M})/mi';
  $re = $reg;
  $subst = '-';
  $str = preg_replace($re, $subst, $str);
  echo "The result of the substitution is " . $str;
}

For test regex online : https://regex101.com/