根据生日获取年龄带单位

function  getAgeByBirthday($birthday){
    if(empty($birthday)) return '';
    //获取当前年份;
    $now_year   = substr(date('Y-m-d'),0,4);

    //获取当前年份月份;
    $now_month  = substr(date('m'),0,1)=='0'  ? substr(date('m'),-1) : date('m');

    //获取当前日期;
    $now_day    = substr(date('d'),0,1)=='0'  ? substr(date('d'),-1) : date('d');

    //获取身份证号对应的年,月份,日期;
    $card_year  = substr($birthday,0,4);  //1986-12-16;
    //获取身份证号对应月份
    $card_month = substr($birthday,5,2);
    $card_month = substr($card_month,0,1)=='0' ? substr($card_month,-1): $card_month;

    //获取身份证号对应日期;
    $card_day   = substr($birthday,8,2);
    $card_day   = substr($card_day,0,1)=='0' ? substr($card_day,-1): $card_day;

    //获取当前年份和身份证年份之差;
    $age         = $now_year  - $card_year;

    //获取当前月份和身份证月份之差;
    $month       = $now_month - $card_month;

    $day         = $now_day   - $card_day;

    $monthArr    = [1,3,5,7,8,10,12];

    $realAge     = '0天';

    if($age < 1 ){ //1岁内
        //比较月份;
        if($month < 1){
            //一个月以内，显示天数;
            if($day > 0 ){
                $realAge  =  $day.'天';
            }
        }elseif($month ==1){
            if($day > 0){
                $realAge  = $month .'个月'. $day.'天';
            }else{
                if(in_array($card_month,$monthArr)){
                    $realAge  = (31+$day).'天';
                }else{
                    $realAge  = (30+$day).'天';
                }

            }
        }else {
            if($day > 0){
                $realAge  = $month .'个月'. $day.'天';
            }else{
                $realAge = ($month-1) .'个月'. (30+$day).'天';
            }
        }

    }elseif($age >=1 && $age <=3 ){ //1-3岁

        if($age ==1 ){
            if($month > 0){
                $realAge    = $age .'岁'.$month.'个月';
            }else{
                $realAge   = (12+$month).'个月';
                if($month==0){
                    if($day < 0){
                        $realAge   = (12-1).'个月';
                    }
                }
            }
        }elseif($age ==3){
            if($month>=0){
                if($day>=0){
                    $realAge     = $age .'岁'.$month.'个月';
                    if($month == 0){
                        $realAge = $age .'岁';
                    }

                }else{
                    $realAge     = $age .'岁'.($month-1).'个月';
                    if($month-1<=0){
                        $realAge = $age .'岁';
                    }
                }
            }else{
                      $realAge  = ($age-1).'岁'.(12+$month).'个月';
            }
        }else{
            if($month > 0){
                $realAge   = $age .'岁'.$month.'个月';
            }else{
                $realAge   = ($age-1).'岁'.(12+$month).'个月';
                if($month == 0){
                    $realAge   = $age .'岁';
                }
            }
        }

    }else{ //大于3岁

        if($now_month == $card_month){
            //比较日期;
            if($now_day >= $card_day){
                $realAge =  $age.'岁';
            }else{
                $realAge =  ($age -1).'岁';
            }
        }elseif($now_month > $card_month){
            $realAge    =  $age.'岁';
        }else{
            $realAge    =  ($age -1).'岁';
        }
    }
    return  $realAge;
}