minimal_rest.php

<?php
/*
Plugin Name: Minimal Rest API
Description: Very minimal DIY REST API plugin scaffold, must refresh permalink
settings to get this to work
*/

/**
 * WP Init Action hook function
 *
 * @param null
 *
 * @return null
 */
function minimal_rest_init() {
        global $wp;

        $wp->add_query_var( 'api' );
}
add_action( 'init', 'minimal_rest_init' );

/**
 * Activation Hook function
 *
 * @param null
 *
 * @return null
 */
function minimal_rest_activate() {
        add_rewrite_rule( '^api/?$',   'index.php?api=/',           'top' );
        add_rewrite_rule( '^api(.*)?', 'index.php?api=$matches[1]', 'top' );

        flush_rewrite_rules();
}
register_activation_hook( __FILE__, 'minimal_rest_activate' );

/**
 * Given a request going to {site_url}/api/{endpoint}, echo the response array in
 * JSON form
 *
 * @param null
 *
 * @return null
 */
function minimal_rest_process_callback() {
        if ( empty( $GLOBALS['wp']->query_vars['api'] ) ) {
                return;
        }

        switch ( $GLOBALS['wp']->query_vars['api'] ) {
        case '/endpoint':
                $content = array('hello' => 'world');
                break;
        default:
                $content = array();
        }

        wp_send_json( $content );
        die();
}
add_action( 'template_redirect', 'minimal_rest_process_callback', PHP_INT_MIN );

/**
 * Logic for relaying REST input
 *
 * Hardly any input validation here. Do not use as-is!
 *
 * @param null
 *
 * @return array
 */
function minimal_rest_populate_data() {
        switch ( $_SERVER['REQUEST_METHOD'] ) {
        case 'POST':
                $return = (array) json_decode( file_get_contents( 'php://input' ), true );
                $return['REQUEST_METHOD'] = 'POST';
        case 'GET':
        case 'PUT':
        case 'DELETE':
                foreach ( $_REQUEST as $key => $value ) {
                        $return[$key] = trim( $value );
                }
                $return['REQUEST_METHOD'] = $_SERVER['REQUEST_METHOD'];
                break;
        default:
        }

        return $return;
}