gistfile1.php

function get_deep_child_category( $categories )
{
	$categories = get_the_category();
    $maxId = 0;
    $maxKey = 0;
    foreach ( $categories as $key => $value )
    {
        if ( $value->parent > $maxId )
        {
            $maxId = $value->term_id;
            $maxKey = $key;
        }
    }
    return $categories[$maxKey];
}

function display_deep_child_category(){
	$deepChild = get_deep_child_category( $categories ); 
	$displayDeepChild = $deepChild->term_id;
	echo $displayDeepChild;
}

function get_deep_child_category()
{
$categories = get_the_category();
$maxId = 0;
$maxKey = 0;
foreach ( $categories as $key => $value )
{
if ( $value->parent > $maxId )
{
$maxId = $value->term_id;
$maxKey = $key;
}
}

return  $categories[$maxKey]->term_id;

}