Snippet to establish a WordPress custom post type in vanilla PHP

mitlib-content-model.php

<?php
/**
 * Plugin Name: MITlib Content Model
 *
 * @package mitlib-content-model
 * @version 0.0.1
 */

/**
 * Register content types
 */
function alert_definition() {
	$labels = array(
		'name'                => _x( 'Alerts', 'Post Type General Name', 'text_domain' ),
		'singular_name'       => _x( 'Alert', 'Post Type Singular Name', 'text_domain' ),
		'menu_name'           => __( 'Alerts', 'text_domain' ),
		'name_admin_bar'      => __( 'Alert', 'text_domain' ),
		'parent_item_colon'   => __( 'Maihaugen', 'text_domain' ),
		'all_items'           => __( 'All Alerts', 'text_domain' ),
		'add_new_item'        => __( 'Add Alert', 'text_domain' ),
		'add_new'             => __( 'New Alert', 'text_domain' ),
		'new_item'            => __( 'New Alert', 'text_domain' ),
		'edit_item'           => __( 'Edit Alert', 'text_domain' ),
		'update_item'         => __( 'Update Alert', 'text_domain' ),
		'view_item'           => __( 'View Alert', 'text_domain' ),
		'search_items'        => __( 'Search Alerts', 'text_domain' ),
		'not_found'           => __( 'No Alerts found', 'text_domain' ),
		'not_found_in_trash'  => __( 'No Alerts found in Trash', 'text_domain' ),
	);
	$args = array(
		'label' => 'Alert',
		'description' => 'Alert',
		'labels'              => $labels,
		'supports'            => array( 'title', 'editor' ),
		'hierarchical'        => false,
		'public'              => true,
		'show_ui'             => true,
		'show_in_menu'        => true,
		'show_in_rest'        => true,
		'menu_position'       => 5,
		'menu_icon'           => 'dashicons-format-gallery',
		'show_in_admin_bar'   => true,
		'show_in_nav_menus'   => true,
		'can_export'          => true,
		'has_archive'         => true,
		'exclude_from_search' => false,
		'publicly_queryable'  => true,
		'capability_type'     => 'post',
		'rewrite'             => array(
			'slug' => 'alert',
		),
	);
	register_post_type( 'alerts', $args );
}
add_action( 'init', 'alert_definition', 0 );


/**
 * Defines start and end date fields (simple text for now)
 */
add_action( 'admin_init', 'admin_init' );
function admin_init() {
	add_meta_box("alert_startdate-meta", "Start date", "alert_dates", "alerts", "side", "low");
}
function alert_dates(){
  global $post;
  $custom = get_post_custom($post->ID);
  $alert_startdate = $custom["alert_startdate"][0];
  $alert_enddate = $custom["alert_enddate"][0];
  ?>
  <label>Start date:</label>
  <input name="alert_startdate" value="<?php echo $alert_startdate; ?>" />
  <label>End date:</label>
  <input name="alert_enddate" value="<?php echo $alert_enddate; ?>" />
  <?php
}

add_action( 'save_post', 'save_details' );
function save_details() {
	global $post;

	update_post_meta( $post->ID, 'alert_startdate', $_POST['alert_startdate'] );
	update_post_meta( $post->ID, 'alert_enddate', $_POST['alert_enddate'] );
}

/**
 * Modifies content list to use custom fields
 */
add_action( 'manage_posts_custom_column', 'alerts_custom_columns' );
add_filter( 'manage_edit-alerts_columns', 'alerts_edit_columns' );
function alerts_edit_columns( $columns ) {
	$columns = array(
		"cb" => '<input type="checkbox" />',
		"title" => 'Alert Headline',
		"started" => 'Start Date',
		"ended" => 'End Date',
	);

	return $columns;
}
function alerts_custom_columns( $column ) {
	global $post;

	switch ( $column ) {
		case "ended":
			$custom = get_post_custom();
			echo $custom["alert_enddate"][0];
			break;
		case "started":
			$custom = get_post_custom();
			echo $custom["alert_startdate"][0];
			break;
	}
}