[Code Related] Checkbox to Dropdown Conversion?

Folks,

I have a Checkbox codes for Categories:

<?php 

  $categories = get_categories( 'orderby=name&hierarchical=0&hide_empty=0' );

  $n = 1;

  foreach ( $categories as $cat ): ?>

  <input type="checkbox" name="postcats[]" value="<?php echo $cat->cat_ID; ?>" >

    <?php 

	echo $cat->cat_name; 

  $n = $n +1;
endforeach; ?><input type="hidden" name="abb_num_cats" value="<?php echo $n;?>">

Observe, the Checked categories are stored in $postcats[].

The Dropdown equivalent of Checkbox code is:

<select name="postcats">
	<?php
	$categories = get_categories( 'orderby=name&hierarchical=0&hide_empty=0' );
  	$n = 1;
    foreach ( $categories as $cat ):
	$cat_id = $cat->cat_ID;

	?>

	<option name="postcats[]" value="<?php echo $cat_id;?>" <?php if ($cat_id==$amaniche_main_category){echo "selected";}?>>

	<?php
	echo $cat->cat_name;
	?>
	</option>
	<?php
    $n = $n +1;
    endforeach; ?>

</select>

My Question is: In the Dropdown codes, how to Store selected option values in Array $postcats[].

I will be so so so Grateful to you, if someone can help me out.