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/