How to echo a PHP Code After the Content

You can try my approach which is used to do your requirement:

function themename_your_content_after($content) {
    if( is_single() ):
    ob_start();
    ?>
     <div class="css">
    <?php if ( should_display_link() ) : ?>
    [subscribe-author-button]
    <?php echo '<a class="btn" style="background-color:#1f1fff; border-radius:100px;" href="'.site_url('chat/').'?new-message&to='.get_the_author_meta( 'user_login' ).'">Chat</a>' ; ?>
    <?php endif; ?>
    [simple-author-box]
    <?php if ($post->post_status == "publish")  { ?>
    <h4>[post-views]</h4>
    <?php   }  else {echo "<span></span>";} ?> 
    <?php if ( should_display_link() ) : ?>
    <a class="btn" href="https://adityaagarwal3.typeform.com/to/Lin6FB" data-mode="popup" style="background-color:#DB4437;color:white;border-radius:25px;" data-hide-headers=true data-hide-footer=true data-submit-close-delay="5" target="_blank">Report Content </a> <script> (function() { var qs,js,q,s,d=document, gi=d.getElementById, ce=d.createElement, gt=d.getElementsByTagName, id="typef_orm_share", b="https://embed.typeform.com/"; if(!gi.call(d,id)){ js=ce.call(d,"script"); js.id=id; js.src=b+"embed.js"; q=gt.call(d,"script")[0]; q.parentNode.insertBefore(js,q) } })() </script>
    <?php endif; ?>
    </div>
    <?php
    $code_content = ob_get_clean();
    $content .= $code_content;
    endif;
    return $content;
}
add_filter('the_content', 'themename_your_content_after');