With PHP, why does option selected not always work?

Question

While I realize that this isn't the most efficient bit of code, it will not work for August or September! I'm using PHP 5.4 and XHTML 1.0 Transitional. I've tested it in the lasted IE, Firefox and Google browsers. I've checked the code that is sent to the browsers and there isn't anything different about August or September. All the other months work fine. When the user first encounters the screen, the stored birthmonth will be extracted from the database and exploded. But August and September will not be selected on the drop down list (but 08 or 09 will be echo-ed to the screen per that bit of code). If the user clicks the drop down list (for month), chooses August or September and clicks the savebtn, August or September will not be selected on the drop down list properly (but 08 or 09 will be echo-ed to the screen per that bit of code). Oh, and when August or September are supposed to be selected, January is selected. Thank you very much ahead of time!

// Basically, if the savebtn was clicked:
$birthmonth = mysqli_real_escape_string($dbc, trim($_POST['birthmonth']));
$birthday = mysqli_real_escape_string($dbc, trim($_POST['birthday']));
$birthyear = mysqli_real_escape_string($dbc, trim($_POST['birthyear']));
$birthdate = $birthyear. '-' . $birthmonth. '-' . $birthday;
echo '<p>The savebtn was clicked. Birthdate: ' . $birthdate . '</p>';
echo '<p>The savebtn was clicked. Birthmonth: ' . $birthmonth . '</p>';

// Basically, if the savebtn was NOT clicked
$birthdate = $row['birthdate'];
if (!empty($birthdate))
list($birthyear, $birthmonth, $birthday) = explode('-', $birthdate);
echo '<p>The savebtn was NOT clicked. DB birthdate: ' . $birthdate . '</p>';
echo '<p>The savebtn was NOT clicked. DB birthmonth: ' . $birthmonth . '</p>';

<form name="usersform" id="usersform" enctype="multipart/form-data" action="<?php echo $_SERVER['PHP_SELF']; ?>"  method="post">
<label for="birthmonth" class="label2">Month:</label>
<select name="birthmonth" id="birthmonth">
<?php
if (!empty($birthmonth) && ($birthmonth == 01))
    echo '<option selected="selected" value="01">January</option>';
else
    echo '<option value="01">January</option>';
if (!empty($birthmonth) && ($birthmonth == 02))
    echo '<option selected="selected" value="02">February</option>';
else
    echo '<option value="02">February</option>';
if (!empty($birthmonth) && ($birthmonth == 03))
    echo '<option selected="selected" value="03">March</option>';
else
    echo '<option value="03">March</option>';
if (!empty($birthmonth) && ($birthmonth == 04))
    echo '<option selected="selected" value="04">April</option>';
else
    echo '<option value="04">April</option>';
if (!empty($birthmonth) && ($birthmonth == 05))
    echo '<option selected="selected" value="05">May</option>';
else
    echo '<option value="05">May</option>';
if (!empty($birthmonth) && ($birthmonth == 06))
    echo '<option selected="selected" value="06">June</option>';
else
    echo '<option value="06">June</option>';
if (!empty($birthmonth) && ($birthmonth == 07))
    echo '<option selected="selected" value="07">July</option>';
else
    echo '<option value="07">July</option>';
if (!empty($birthmonth) && ($birthmonth == 08))
    echo '<option selected="selected" value="08">August</option>';
else
    echo '<option value="08">August</option>';
if (!empty($birthmonth) && ($birthmonth == 09))
    echo '<option selected="selected" value="09">September</option>';
else
    echo '<option value="09">September</option>';

if (!empty($birthmonth) && ($birthmonth == 10))
    echo '<option selected="selected" value="10">October</option>';
else
    echo '<option value="10">October</option>';
if (!empty($birthmonth) && ($birthmonth == 11))
    echo '<option selected="selected" value="11">November</option>';
else
    echo '<option value="11">November</option>';
if (!empty($birthmonth) && ($birthmonth == 12))
    echo '<option selected="selected" value="12">December</option>';
else
    echo '<option value="12">December</option>';
?>
</select><br />
<input type="submit" class="submitbtn" name="savebtn" id="savebtn" value="Save" />

Answer 1

Leading 0s on numbers make PHP interpret them as octal. 08 and 09 are not valid octal numbers. Get rid of the leading zeros.

Answer 2

To provide an example of the ternary I was suggesting in the comment:

echo '<option value="01"'. !empty($birthmonth) && ($birthmonth == 1) ? ' selected="selected"' : '' .'>January</option>';

Your IDE complaining about PHP and option tags may be something else. PHP is only echoing the option tag to the browser and xhtml transitional supports option, so there shouldn't be a problem in the basic construction. You should post your output html in your question for further debugging if it is still complaining after these updates have been applied.

This answer is just to elaborate on a comment. It is not to be treated as an official answer, though the example also has the correction mentioned by john applied.