Find ud af om det er skudår

<?php 
    if(date('m') == 2) {
    $day_start  = 1;
    $day_end = 29;	
	} else {
    $day_start  = 1;
    $day_end = 31;
    }
    $user_selected_day = date("d"); // user date of birth year
    echo '<select id="day" name="year" class="form-control">'."\n";
	echo '<option value="0">Måned</option>';
    for ($i_day = $day_start; $i_day <= $day_end; $i_day++) {
        $selected = ($user_selected_day == $i_day ? ' selected' : '');
        echo '<option value="'.$i_day.'"'.$selected.'>'.$i_day.'</option>'."\n";
    }
    echo '</select>'."\n";
?>