To get all terms of custom taxonomy in WordPress we can use get_terms() function. It retrieves the terms in a given taxonomy or list of taxonomies. Here is an example:

<?php 
          $terms = get_terms( array(
               'post_type' => 'portfolio',
               'taxonomy' => 'portfolio_category',
               'hide_empty' => false,
               ) );
            ?>
            <select class="form-control" name="portfolio_cat" id="parent_cat">
                 <option value="">Select Category</option>
                    <?php  
                        foreach($terms as $term) {
                            ?>
                                <option value="<?php echo $term->term_id; ?>">
                                        <?php echo $term->name; ?>
                                </option>
                            <?php 
                        }   
                            
                    ?>      
                </select>