Helper function to calculate renaming days in birthday. If birthday is already passed then function calculate days according to next coming year.

get_birthday_days.php

function get_birthday_days($birthday)
{
    $cur_day = date('Y-m-d');
    $cur_time_arr = explode('-', $cur_day);
    $birthday_arr = explode('-', $birthday);
    $days=0;
    $cur_year_b_day = $cur_time_arr[0] . "-" . $birthday_arr[1] . "-" . $birthday_arr[2];

    if (strtotime($cur_year_b_day) < time()) {
        $cur_year_b_day = date('Y', strtotime('+1 year')) . "-" . $birthday_arr[1] . "-" . $birthday_arr[2];
        $diff = strtotime($cur_year_b_day) - time();
        $days = floor($diff / (60 * 60 * 24));
       
    } else {
        $diff = strtotime($cur_year_b_day) - time();
        $days = floor($diff / (60 * 60 * 24));
       
    }
    
     return $days;
}