为每个创建3个selectbox（月，日，年）php循环选项，并在`months`更改时更改`days`的选项

I have these select boxes as follow:

<?php 
echo'
<select id="months"></select>

<select id="days"></select>

<select id="years"></select>
'; ?>

I want the options for each select boxes to be the current date to be automatically selected on load. Like for example today(January 28, 2016): the first select box should have January selected. and the next one should be 18, and the last 1 should be 2016.

Actually I already got that working for the days select box with this code.

for ($x = 1; $x <= 31; $x++) {
        echo '<option value="'.$x.'" ';

        if ($x == date_format($nowdate,'d'))
        {
            echo 'selected';
        }
        echo' >'.$x.'</option>';
        }

But I don't know how to do it on months. And 1 more problem, I want when months changes, the days should also change its options depending on the number of days the selected month has. Any help would be greatly appreciated. Thanks.

Consider 2000-3000 as options included for years Select box

You can try the below code for populating months :

    for ($x = 1; $x <= 12; $x++) {
        $mon = date('F', mktime(0,0,0,$x));
        echo '<option value="'.$mon.'" ';
        if ($mon == date('F'))
        {
           echo 'selected';
        }
        echo' >'.$mon.'</option>';
    }

Same way, you can use date('Y') to figure out the current year.

For the JavaScript part, you can use the following code -

Onchange of months select box call the following function and pass two parameters (current selected month and selected year) -

    function getCurrentdays(month,year) {
        var currentdays = new Date(year, month, 0).getDate();   
        populatedayscombo(currentdays);
    }

    function populatedayscombo(currentdays)
    {
        document.getElementById("monthscombo").options.length = 0;
        for (x = 1; x <= currentdays; x++) 
        {
            var newOption = document.createElement("option");
            newOption.text = x;
            newOption.value = x;
            document.getElementById("monthscombo").add(newOption);
        }
    }