get a category from a post code example

Example 1: how to get category from post id

get_the_category($post->ID)

Example 2: get post by category

<section class="films-tabs pd-40" id="portfolio">

  <?php $film_genres = get_terms('portfolio_cat'); // get all the genres ?>

<div class="container">
<div class="row">
  <!-- Nav tabs -->
  <ul class="nav nav-tabs nav-justified">
    <?php foreach($film_genres as $film_genre) { ?>
      <li>
        <a href="#<?php echo $film_genre->slug ?>" data-toggle="tab"><?php echo $film_genre->name ?></a>
      </li>
    <?php } ?>
  </ul>
</div>
</div>

  <!-- Tab panes -->
  <div class="tab-content">

    <?php foreach($film_genres as $film_genre) { ?>

      <div class="tab-pane" id="<?php echo $film_genre->slug ?>">
        <?php 	
        $args = array(
          'post_type' => 'portfolio',
          'showposts' => -1,
          'orderby' => 'title',
          'order' => 'ASC',
          'tax_query' => array(
            array(
              'taxonomy' => 'portfolio_cat',
              'field' => 'slug',
              'terms' => $film_genre->slug
            )
          )
        );
        $films = new WP_Query( $args );		
        ?>

        <?php if ( $films->have_posts() ) : // make sure we have films to show before doing anything?>
        <div class="table">
            <div class="container">
                <div class="row">
                  <?php while ( $films->have_posts() ) : $films->the_post(); ?>	
                  
                    <div class="table-cont"><a data-fancybox="gallery" href="<?php echo get_the_post_thumbnail_url();?>".><?php the_post_thumbnail() ?></a></div>
                  
                  <?php endwhile; ?>
                  <?php wp_reset_query() ?>
                 </div>
            </div>
        </div>
        <?php endif; ?>

      </div>
    <? }  ?>

  </div><!-- tab-content -->

</section><!-- film-tabs -->