WordPress rewrite tutorial

pmg-rewrite.php

<?php
/*
Plugin Name: Rewrite Rule Tutorials
*/

add_action( 'init', 'pmg_rewrite_add_rewrites' );
function pmg_rewrite_add_rewrites()
{    
    add_rewrite_endpoint( 'json', EP_PERMALINK );
    add_rewrite_rule(
        '^p/(\d+)/?$', // p followed by a slash, a series of one or more digets and maybe another slash
        'index.php?p=$matches[1]',
        'top'
    );
    
    add_rewrite_tag( '%form%', '[^/]' );
    add_rewrite_rule(
        '^submit/?$',
        'index.php?form=true',
        'top'
    );
    add_feed( 'json', 'pmg_rewrite_json_feed' );
}

function pmg_rewrite_json_feed()
{
    $posts = get_posts();
    $out = array();
    foreach( $posts as $p )
    {
        $out[] = array(
            'title' => $p->post_title,
            'content' => $p->post_content
        );
    }
    header('Content-Type: text/plain');
    echo json_encode( $out );
}


register_activation_hook( __FILE__, 'pmg_rewrite_activation' );
function pmg_rewrite_activation()
{
    pmg_rewrite_add_rewrites();
    flush_rewrite_rules();
}

add_action( 'template_redirect', 'pmg_rewrite_catch_form' );
function pmg_rewrite_catch_form()
{
    if( is_singular() && get_query_var( 'json' ) )
    {
        $post = get_queried_object();
        $out = array(
            'title'     => $post->post_title,
            'content'   => $post->post_content
        );
        header('Content-Type: text/plain');
        echo json_encode( $out );
        exit();
    }
}


//add_filter( 'query_vars', 'pmg_rewrite_add_var' );
function pmg_rewrite_add_var( $vars )
{
    $vars[] = 'form';
    return $vars;
}

add_filter( 'request', 'pmg_rewrite_filter_request' );
function pmg_rewrite_filter_request( $vars )
{
    if( isset( $vars['json'] ) ) $vars['json'] = true;
    return $vars;
}