Generating a selection box for the day of the month

Question

 <select name="checkInDay" tabindex="3" onchange="calcDay();" class="ffe selectform" id="checkInDate">
    <?php for($i=1;$i<=31;$i++) {
        $value = $i < 10 ? "0".$i : $i;
     ?> 
     <option value="<?= $value ?>" <?php if($i == 12) {?> selected="selected" <?php } ?> > <?= $i ?> </option>
     <?php } ?>

  </select>

Answer 1

I suggest to build a function which can be reused for the options:

function select_options($options, $selected = null) {
  $_options = '';
  foreach ($options as $value => $content) {
    $_options .= sprintf("<option value=\"%s\"%s>%s</option>\n", $value, $value == $selected ? ' selected="selected"' : '', $content);
  }

  return $_options;
}

And some PHP "magic" for the values generation but it is not really needed:

<?php $values = array_map(create_function('&$v', 'return sprintf("%02d", $v);'), range(1, 31)) ?>
<select name="checkInDay" tabindex="3" onchange="calcDay();" class="ffe selectform" id="checkInDate">
  <?php echo select_options(array_combine($values, range (1, 31)), 12) ?>
</select>

But more important than all: in a HTML template, use the PHP alternative notation.

Answer 2

Remove Magic Numbers

What does "12" represent? Why is it the selected value?

Rather than leaving this as-is, I would strongly recommend replacing it with a more descriptive constant.

Business Logic

This may or may not be something worth changing. It really depends on what this drop-down is being used for.

Does it make sense for this drop-down to have 31 numbers listed?

It seems like the option 31 would only make sense for some months, whereas the options 1 - 28 make sense for all months. Is this being used next to Month drop-down? If so, perhaps it should dynamically update based on the month selected. If not, perhaps the options should be from 1 - 28, and then a last option, "Last Day of Month".

If you keep it as-is, just be sure to be clear to your users what will happen in a month like February if they select 30.

Short Tags

I would personally avoid the use of short tags.

For more information, see https://stackoverflow.com/questions/200640/are-php-short-tags-acceptable-to-use

Text Formatting

To pad your number with leading zeros, use a built-in function.

As @spookycoder suggested:

$value = sprintf("<option value=\"%02d\"", $i);

Code Is Read Far More Than It Is Written

Make the code as clear as is possible. As @Keven recommended, use the PHP alternative notation.

A Matter of Taste

While this is just my preference, I would also consider moving whether or not the option is selected off the HTML option line.

<?php 
    if ($day == DEFAULT_CHECK_IN_DAY):  // Changed $i to $day and made 12 a const for clarity
        $selected = ' selected="selected"';
    else:
        $selected = '';
    endif;
?>
<option value="<?php echo sprintf("%02d", $day) ?>"<?php echo $selected ?>>
    <?php echo $day ?>
</option>