Creating multiple page URL without creating the pages in WordPress

This is a simple outline so don’t use this in production but it’s a working example.

Define your template and place it in templates/template-portfolio-template.php inside your theme.

<?php
/*
 * Template Name: Portfolio Template
 * Description: Page template to display portfolio custom post types
 */

echo 'This is the Portfolio Template!';

Next, you’ll need to programmatically generate the posts.

function programmatically_create_post($post_type="post", $title, $content, $template_rel_path="")
{
    // Initialize the page ID to -1. This indicates no action has been taken.
    $post_id = -1;

    // Prep
    $author_id = get_current_user_id();
    $title = wp_strip_all_tags( $title ); // remove any junk
    $slug = sanitize_title ($title); // converts to a usable post_name
    $post_type = post_type_exists($post_type) ? $post_type : 'post'; // make sure it exists

    // If the page doesn't already exist, then create it
    if( null == get_page_by_title( $title ) ) {

        // Set the post ID so that we know the post was created successfully
        $post_id = wp_insert_post( 
          array(
          'post_name'           =>  $slug,
          'post_title'          =>  $title,
          'post_content'        =>  $content,
          'post_type'           =>  $post_type,
          'post_author'         =>  $author_id,
          'comment_status'      =>  'closed',
          'ping_status'         =>  'closed',
          'post_status'         =>  'publish',
          )
        );

    if( $post_id && $post_id > 0 && ! empty( $template_rel_path)) {

      // make sure the template exists
      $template_full_path = trailingslashit( get_stylesheet_directory() ) . $template_rel_path;
      if( file_exists( $template_full_path )) {

         // set the post meta data -- use relative path
         update_post_meta( $post_id, '_wp_page_template', $template_rel_path );
      }
    }

    // Otherwise, we'll stop
    } else {
            // Arbitrarily use -2 to indicate that the page with the title already exists
            $post_id = -2;

    } // end if

  return $post_id;

} // end programmatically_create_post

Then test that it works w/or without a template.

add_action( 'init', function(){

  $template="templates/template-portfolio-template.php";

  $post_id_temp = programmatically_create_post( 'page', 'My Page With a Template', 'This is some sample content for the template! ' . timer_stop(4), $template );

  $post_id_no_temp = programmatically_create_post( 'page', 'My Page Without A Template', 'This is some sample content! ' . timer_stop(4), '' );

  if( -1 == $post_id_temp || -2 == $post_id_temp ) { // The post wasn't created or the page already exists
     //...
  } else {
     $post = get_post($post_id_temp);
     $url = get_permalink($post->ID); // your new page url
  }

  if( -1 == $post_id_no_temp || -2 == $post_id_no_temp ) { // The post wasn't created or the page already exists
     //...
  } else {
     $post = get_post($post_id_no_temp);
     $url = get_permalink($post->ID); // your new page url 
  }

} );