Question

I am adding a feature to edit the ads a user puts. For that I want to make the value selected which is selected by the user when he entered the values . How can I implement this?

<div class="nm">
  Category
</div>
<div class="fl">
  <select name="category" id = "ad_category" onchange="cangeSubCat(this.value)" >
    <option value="">---Select Category---</option>
    <option value="real_estate">Real Estate</option>
    <option value="hotels_resturants">Hotels and Resturants</option>
    <option value="car">Car Rental</option> 
  </select>
</div>

Answer 1

Add the selected HTML <option> attribute, in XHTML it is selected="selected".

<option value="value" selected>title</option>
                      ^^^^^^^^

Documentation: http://www.w3.org/TR/html4/interact/forms.html#h-17.6.1

I mean how will I know which one should be selected..it will be different for different users . I wnt it to be selected which user selects during adding the ad

That depends on your code. You can do this with an array with all value/title pairs (value => title) and the value which is selected. Then when key (value) equals the selected one, the attribute is added in output. As it’s an array it’s easy to iterate over it.

$otpions = array(
    'a' => 'A',
    'b' => 'B',
);
$selected = 'b';

echo '<select>';
foreach ($options as $value => $title)
{
    printf('<option value="%s" %s>%s</option>', 
           $value, $selected == $value ? 'selected' : '', $title);
}
echo '</select>';