２つの日付から日数を算出

diff_day.php

function diff_day($date1, $date2) {
    
    $num1 = strtotime($date1);
    $num2 = strtotime($date2);
    
    $num_diff = $num2 - $num1;
    
    $day_diff = $num_diff / (60 * 60 * 24);
    
    return $day_diff;
}