JQuery / PHP：将选择框替换为另一页上的另一个选择框

On my main page, I have a simple dropdown box:

<select name="miles" id="miles">
<option value="5">5</option>
<option value="10">10</option>
<option value="15">15</option>
<option value="25">25</option>
<option value="50">50</option>
<option value="75">75</option>
<option value="100">100</option>
</select>
<button type="button" name="button" id="button">Search</button>

When this button is clicked, it performs a JQuery $.load:

$("#miles").load("main_data.php #miles_data");

And here the page button is calling, main_data.php:

<?php

... code to calculate $b

?>

<span id="miles_data">
<option value="5" <?php if ($b == 5) { echo "selected"; } ?>>5</option>
<option value="10" <?php if ($b == 10) { echo "selected"; } ?>>10</option>
<option value="15" <?php if ($b == 15) { echo "selected"; } ?>>15</option>
<option value="25" <?php if ($b == 25) { echo "selected"; } ?>>25</option>
<option value="50" <?php if ($b == 50) { echo "selected"; } ?>>50</option>
<option value="75" <?php if ($b == 75) { echo "selected"; } ?>>75</option>
<option value="100" <?php if ($b == 100) { echo "selected"; } ?>>100</option>
</span>

My goals seem simple: Execute a $.load to another page, calculate an integer and set it to a variable, select the matching dropdown item and return it to the main page.

With my code above, the dropdown box on the main page becomes blank. Does anyone notice a solution?

One possible issue is that the format for a selected 'option' is:

<select>
  <option>Volvo</option>
  <option selected="selected">Saab</option>
  <option>Mercedes</option>
  <option>Audi</option>
</select>

Blatantly ripped from here

So you want to have

<option value="5" <?php if ($b == 5) { echo 'selected="selected"'; } ?>>5</option>