var showDiffDate = function( tYear, tMonth, tDay) {
    var nowDate = new Date();
    var dnumNow = nowDate.getTime();
    var targetDate = new Date( tYear, tMonth-1, tDay );
    var dnumTarget = targetDate.getTime();

    var diffMSec = dnumTarget - dnumNow;
    var diffDays = diffMSec / ( 1000 * 60 * 60 * 24 );
    var showDays = Math.ceil( diffDays ); 
    var Msg;
    if( showDays >= 0 ) {
       Msg = "" + showDays + " days(" + function(){
         var is_days = parseInt(parseInt(showDays) - Math.floor(showDays / 7) * 2);
         is_days = (nowDate.getDay() === 0)?is_days -2:is_days;
         is_days = (nowDate.getDay() === 6)?is_days -3:is_days;
         return is_days;
       }() +")";
    }
    else {
       Msg = "" + (showDays * -1) + "";
    }
    return Msg;
 };

 document.getElementById("days").innerText = showDiffDate(2022,7,15);

<?php
header('Access-Control-Allow-Origin: *');
date_default_timezone_set('Asia/Tokyo');
$birth_date = (string)$_POST["birth_date"];
$max_age = (int)$_POST["point_age"];

/**
 * @param string $birth_date
 * @return string|false
 */
function check1($birth_date = ""): bool
{
    $flg = false;
    $str_date = explode("/", $birth_date);
    if (count($str_date) === 3) {
        $flg = true;
        if (!((int)$str_date[0] >= 1000)) {
            $flg = false;
        }
        if(((int)$str_date[0] > (int)date("Y"))){
            $flg = false;
        }
        if (!((int)$str_date[1] >= 1 && (int)$str_date[1] <= 12)) {
            $flg = false;
        }
        if ($flg) {
            if ((int)$str_date[1] === 2) {
                if (!((int)$str_date[2] >= 1 && (int)$str_date[2] <= 28)) {
                    $flg = false;
                }
                if ((int)$str_date[0] % 4 === 0) {
                    $flg = true;
                    if (!((int)$str_date[2] >= 1 && (int)$str_date[2] <= 29)) {
                        $flg = false;
                    }
                    if ((int)$str_date[0] % 100 === 0) {
                        $flg = true;
                        if (!((int)$str_date[2] >= 1 && (int)$str_date[2] <= 28)) {
                            $flg = false;
                        }
                        if ((int)$str_date[0] % 400 === 0) {
                            $flg = true;
                            if (!((int)$str_date[2] >= 1 && (int)$str_date[2] <= 29)) {
                                $flg = false;
                            }
                        }
                    }
                }
            } else {
                $last_day = [4, 6, 9, 11];
                if (array_search((int)$str_date[1], $last_day, false)!== false) {
                    if (!((int)$str_date[2] >= 1 && (int)$str_date[2] <= 30)) {
                        $flg = false;
                    }
                } else {
                    if (!((int)$str_date[2] >= 1 && (int)$str_date[2] <= 31)) {
                        $flg = false;
                    }
                }
            }
        }
    }
    return $flg;
}

/**
 * @param int $age
 * @return string|false
 */
function check2($age = 0): bool
{
    $flg = true;
    if ($age < 0) {
        $flg = false;
    }
    return $flg;
}


/**
 * @param string $birth_date
 * @param string $maxage
 * @return string $reslut
 */
function sumup($birth_date, $maxage)
{
    $reslut = [];
    $birth_date_array = explode("/", $birth_date);
    $birth_date = sprintf("%04d%02d%02d", $birth_date_array[0], $birth_date_array[1], $birth_date_array[2]);
    $today = date('Ymd');
    $age = floor(($today - $birth_date) / 10000);
    $day1 = new DateTime("{$birth_date_array[0]}-{$birth_date_array[1]}-{$birth_date_array[2]}");
    $day2 = new DateTime();    
    $interval1 = $day1->diff($day2, true);
    $baseday =  (int)($interval1->format('%a'));
    if ((int)$maxage <= (int)$age) {
        $pointday = 0;
    } else {
        $maxage--;
        $day3 = new DateTime((date('Y') + ($maxage - $age)) . "-{$birth_date_array[1]}-{$birth_date_array[2]}");
        $interval2 = $day2->diff($day3, true);
        $pointday = (int)($interval2->format('%a'))+1;
    }


    $reslut = [
        [
            "result" => "success",
            "age"=>$age ."歳",
            "baseday" => $baseday . "日(生きてきた日数)",
            "pointday" => $pointday . "日(" .($maxage +1). "歳まであと)"
        ]
    ];
    return json_encode($reslut);
}

if (!check1($birth_date)) {
    print json_encode([
        [
            "result" => "error",
            "error" => "string is invalid1"
        ]
    ]);
} elseif (!check2($max_age)) {
    print json_encode([
        [
            "result" => "error",
            "error" => "string is invalid2"
        ]
    ]);
} else {
    print sumup($birth_date, $max_age);
}