Add shortcode [keywords] to a post, to list all keywords found in the content of a post. Keywords can be added to the post in which the shortcode is added by creating custom field(s) named 'keyword'. Keywords can be defined using the shortcode attribute 'words' (ex: [keywords words=apples,oranges] ). Post ID can be specified using the shortcode …

post-keywords-shortcode.php

<?php
/**
 * Plugin Name: Post Keywords Shortcode
 * Plugin URI: https://gist.github.com/sscovil/5779312
 * Description: Searches a post for keywords and displays them if found.
 * Version: 0.1
 * Author: Shaun Scovil
 * Author URI: http://shaunscovil.com
 * License: GPL2
 */
 
function post_keywords_shortcode_06132013( $atts ) {
    global $post;
    $post_id  = ( isset( $atts['id'] ) ) ? $atts['id'] : $post->ID;
    $content  = get_the_title( $post_id ) . get_the_content_by_id( $post_id );
    $content  = preg_replace( '/[keyword*]/', '', $content );
    $content  = apply_filters( 'the_content', $content );
    $keywords = ( isset( $atts['words'] ) ) ? explode( ',', $atts['words'] ) : get_post_meta( $post_id, 'keyword' );
    $output   = '';
    $prefix   = 'Keywords: ';
    foreach( $keywords as $keyword ) {
        if ( false !== stripos( $content, $keyword ) ) {
            $output .= $prefix . $keyword;
            $prefix  = ', ';
        }
    }
    return $output;
}
add_shortcode( 'keywords', 'post_keywords_shortcode_06132013' );
 
if ( function_exists( 'get_the_content_by_id' ) ) {
    function get_the_content_by_id( $post_id ) {
        global $post;
        if ( $post->ID == $post_id )
            return get_the_content();
    
        $findpost = get_post( $post_id );
        return $findpost->post_content;
    }
}