$arrayLetters = preg_split('//u', "оди́н", NULL, PREG_SPLIT_NO_EMPTY);
$arrayLettersResult = [];
$stressSym = "́";
foreach ($arrayLetters as $i => $letter) {
if ($letter == $stressSym) continue;
if (!empty($arrayLetters[$i + 1]) && $arrayLetters[$i + 1] == $stressSym) {
$arrayLettersResult[] = $letter . $stressSym;
} else {
$arrayLettersResult[] = $letter;
}
}
var_dump($arrayLettersResult);
/*
array(4) {
[0]=>
string(2) "о"
[1]=>
string(2) "д"
[2]=>
string(4) "и́"
[3]=>
string(2) "н"
}
*/