kaibun_check.php

<?php
$str = "よのなかねかおかおかねかなのよ";
var_dump(kaibun_check($str));

function kaibun_check($str) {
    $char_code = 'utf8';
    $length = mb_strlen($str, $char_code);
    for ($i = 0; $i < $length / 2; $i++) {
        if (mb_substr($str, $i, 1, $char_code) !== mb_substr($str, $length - $i - 1, 1, $char_code)) {
            return false;
        }
    }
    return true;
}

% php kaibun_check.php
bool(true)