add page to singe.php

do a thumbnail, or a link or a (you say one page) cut and paste.
If it is a page why would you want it as a temporary post. maybe
to show how it looked at a specific time. if it is a page then just call
it a static page. try embedding it in the post. still doesn’t sound right.

have 500 posts with small seo content and i want to show my productpage under each post. I can ofcourse copy my produktspage and make a post of it. It must be a commando get_post…id123 i can cut in in single.php

Use a filter on the_content() to add content below the content of a pos

https://codex.www.ads-software.com/Plugin_API/Filter_Reference/the_content

so in your filter, you’d get the content of that post, format it up into a string, and return $content . $addtional_content

so what can that string look like? I′m not so good in php…..

add_filter( 'the_content', 'featured_image_before_content' ); 

 function featured_image_before_content( $content ) {
    if ( is_singular('post') && has_post_thumbnail()) {
        $thumbnail = get_the_post_thumbnail();

        $content = $thumbnail . $content;

		}

    return $content;
}

i want to narrow it down do one specific post. How to do that?

I think this is closer to what you want.

add_filter( 'the_content', 'featured_image_before_content' ); 

 function featured_image_before_content( $content ) {
    $post_to_get = 123; // change to your target post
    if ( get_the_ID() == $post_to_get ) {
       return $content;
       }
    if ( is_singular('post') {
        $add_post = get_post( $post_to_get );
        $addl_content = $addl_post->post_content;
        $addl_content = apply_filters( 'the_content', $addl_content );
        $thumbnail = get_the_post_thumbnail( $addl_post);
        $content = $thumbnail . $addl_content;
		}
    return $content;
}

Thanks, but i cant get it….

<?php

get_header(); ?>

    <div id="single_page_wrap" class="clearfix">

        <div class="container">
            <div class="row">

<?php while (have_posts()) : the_post();?>

    <div class="col-md-8 col-sm-8 single_page_content">
                    <div class="single_title_image clearfix">
                        <h1><?php the_title(); ?></h1>
                        <?php the_post_thumbnail('full'); ?>
                    </div>
                    <!--single_title_image clearfix-->

                    <div class="single_content_text clearfix">
                        <?php
                        the_content();
                        ?>

//start stern data 

add_filter( 'the_content', 'featured_image_before_content' ); 

 function featured_image_before_content( $content ) {
    $post_to_get = 6461; // change to your target post
    if ( get_the_ID() == $post_to_get ) {
       return $content;
       }
    if ( is_singular('post') {
        $add_post = get_post( $post_to_get );
        $addl_content = $addl_post->post_content;
        $addl_content = apply_filters( 'the_content', $addl_content );
        $thumbnail = get_the_post_thumbnail( $addl_post);
        $content = $thumbnail . $addl_content;
		}
    return $content;
}

// end of stern data 

                    </div>
                    <!--clearfix-->
                </div>
                <!--col-md-8 single_page_content-->
<?php endwhile; ?>
                <?php wp_reset_query(); ?>
                <div class="col-md-4 col-sm-4 single_wid_wrap">
                    <?php get_sidebar('post') ?>

                </div>
                <!--col-md-4 single_wid_wrap-->

            </div>
            <!--row-->
        </div>
        <!--container-->

    </div><!--single_page_wrap-->

<?php get_footer(); ?>

Cant see post 6461 under my post. The code shows as text inside the content. I attach single.php for input,