How To Create Custom WordPress Ajax Endpoint That Works For Both Admins and Visitors

functions--example-wp_ajax_-wp_ajax_nopriv_-actions.php

<?php
// Add wp_ajax_ and wp_ajax_nopriv_ actions
add_action('wp_ajax_locations', 'wp_ajax_get_locations');
add_action('wp_ajax_nopriv_locations', 'wp_ajax_get_locations');

// Get locations function
function wp_ajax_get_locations(){

	// Latitude and longitude (defaults to Dayton, Ohio)
	$lat = isset($_POST['lat']) ? $_POST['lat'] : 39.758949;
	$lng = isset($_POST['lng']) ? $_POST['lng'] : -84.191605;
      
	// Query all locations
	$locations = new WP_Query([
		'post_type' 	 => 'location',
		'numberposts' 	 => -1,
		'posts_per_page' => -1,
	]);
  
	// Locations JSON array
	$locations_json = [];
	if($locations->have_posts()){
		while($locations->have_posts()){
			$locations->the_post();
      
			// Get custom ACF fields
			$title = get_field('title');
			$latitude = get_field('latitude');
			$lngitude = get_field('longitude');
			$address = get_field('address');
			$phone = get_field('phone');
      
			// Create new array location item
			$locations_json[] = [
				'title' => $title,
				'latitude' => (float) $latitude,
				'longitude' => (float) $lngitude,
				'address' => $address,
				'phone' => $phone,
			];
		}
	} wp_reset_postdata(); wp_reset_query();
  
	// Order the locations from closest to furthest based on user latitude and longitude
	usort($locations_json, function($a, $b) use ($lat, $lng){
		return pow($a['latitude'] - $lat, 2) + pow($a['longitude'] - $lng, 2)
		> pow($b['latitude'] - $lat, 2) + pow($b['longitude'] - $lng, 2) ? 1 : -1;
	});
  
	// Echo the JSON
	header('HTTP/1.1 200 OK');
	header('Content-Type: application/json');
	echo json_encode($locations_json);
  
	// Be sure to stop execution afterward.
	wp_die();
}