How to get category id of current post?

function catName($cat_id) {
    $cat_id = (int) $cat_id;
    $category = &get_category($cat_id);
    return $category->name;
}
function catLink($cat_id) {
    $category = get_the_category();
    $category_link = get_category_link($cat_id);
    echo $category_link;
}

function catCustom() {
   $cats = get_the_category($post->ID);
    $parent = get_category($cats[1]->category_parent);
    if (is_wp_error($parent)){
        $cat = get_category($cats[0]);
      }
      else{
        $cat = $parent;
      }
    echo '<a href="'.get_category_link($cat).'">'.$cat->name.'</a>';    
}

USE <a href="https://wordpress.stackexchange.com/questions/247859/<?php catLink(1); ?>"> <?php catName(1); ?>