废话不多说，上代码：
 

function ishuiwen($str)
{
    $l = 1;
    //全是中文
    if (preg_match("/^[\x7f-\xff]+$/", $str)) {
        $len = mb_strlen($str);
        $k = intval($len / 2) + 1;
        for ($j = 0; $j < $k; $j++) {
            if (substr($str, 3 * $j, 3) != substr($str, -(3 * ($j + 1)), 3)) {
                $l = 0;
                break;
            }
        }

    } //全不是中文
    else {
        $len = strlen($str);
        $k = intval($len / 2) + 1;
        for ($j = 0; $j < $k; $j++) {
            if (substr($str, $j, 1) != substr($str, $len - $j - 1, 1)) {
                $l = 0;
                break;
            }
        }

    }

    if ($l == 1) {
        return true;
    } else {
        return false;
    }
}

//$str = '上海自来水来自海上';
//$str = '12321';
$str = 789987;
var_dump(ishuiwen($str));