Link title to external link instead of permalink - php

Question

I'm trying to get the same effect as the plugin Pages Link To where the title of the post is linked to an external link. The reason I don't want to use the plugin is the link is generated dynamically when the post is saved, but I'm unable to update the the permalink of the post with the external link.
Below is my code in functions.php:
function savepost( $post_id ) {
if( $_POST['post_type'] == 'books' ){
$genre = strip_tags(get_field('genre'));
$author = strip_tags(get_field('author'));
$extlink = "http://www.".$genre."/".$author.".com";
update_post_meta( $post_id, 'extlink', $extlink);
$url = strip_tags(get_field('extlink',$post));
update_post_meta( $post_id, 'post_link', $url);
}
}
add_action( 'save_post', 'savepost' );
i m trying another method in which i assigned a template to the post so that when the post loads it redirects to the link but it doesn't redirect
my code
<?php
ob_start();
?>
<?php
/**
* Template Name: post Redirection Template
*/
get_header();
$redirecturl = strip_tags(get_field('extlink',$post));
wp_redirect($redirect_url);
get_sidebar();
get_footer();
?>
<?php
ob_end_flush();
?>

Answer 1

What you should do, is insert the external link as a custom field in the post editor, then display the custom field value in place of the_permalink(). You could use a plugin such as Advanced Custom Fields to grab the URL from the custom field.
EDIT 1: More clarification using the Advanced Custom Fields plugin as an example. The field name for this example is url.
You should use this wherever you want the custom permalink to appear throughout your site, such as in your archive.php, category.php, etc. Replace the code that looks something like this:
<?php the_title(); ?>
with this:
<?php
$value = get_field( "url" );
if( $value ) { ?>
<?php the_title(); ?>
<?php } else { ?>
<?php the_title(); ?>
<?php } ?>
EDIT 2: Clarifying additional information.
You can add a function to the header.php of your theme that checks if the url is set, then redirects to the external link that way, if your user goes directly to the permalink, it will still redirect them. In fact, you could use this code without using the above code to display the external link.
<?php
$value = get_field( "url" );
if( $value ) {
header('Location: '.$value);
die();
} else {} ?>
Warning: make sure to use this code before any HTML (or text) has been passed to the browser, or it will not work correctly.