プラグインを使わずにHTTPS接続で運用されているWordpressの記事内にLeafletで地図を表示させるためのショートコードのサンプル。

simple_leaflet_demo.php

<?php
/*
このコードを使用した結果に関しては、いかなる保証も行いませんので、
その点につきましてはあしからずご了承ください。
*/
$_simple_leaflet_demo_map_number = 0;

function simple_leaflet_demo($atts) {
  global $_simple_leaflet_demo_map_number;
  $atts = shortcode_atts(array(
     "lat" => 35.510281,
     "lon" => 139.606164,
     "zoom" => 16,
     "popup" => "International Stadium Yokohama<br>横浜国際総合競技場"
  ),$atts);
  $str = "";
  
  if ($_simple_leaflet_demo_map_number == 0) {
     $str = "<script type=\"text/javascript\">
     function addOnloadEvent(f) {
     	if ( typeof window.addEventListener != \"undefined\" )  
    	   window.addEventListener( \"load\", f, false );  
  	else if ( typeof window.attachEvent != \"undefined\" ) {  
    	   window.attachEvent( \"onload\", f );  
  	} else {
	   window.onload = f;
	}
     }

     var head = document.getElementsByTagName('head')[0];
     var link = document.createElement('link');
     link.href = 'https://unpkg.com/leaflet@1.2.0/dist/leaflet.css';
     link.rel = 'stylesheet';
     head.appendChild(link);

     var script = document.createElement('script');
     script.src = 'https://unpkg.com/leaflet@1.2.0/dist/leaflet.js';
     script.async = false;
     head.appendChild(script);
</script>";
  }
  $_simple_leaflet_demo_map_number++;
  $str .= "<div id=\"mapid".$_simple_leaflet_demo_map_number."\" style=\"height:250px\"></div>
<script type=\"text/javascript\">
onload_event_".$_simple_leaflet_demo_map_number." = function(){
var map = L.map('mapid".$_simple_leaflet_demo_map_number."').setView([".$atts["lat"].", ".$atts["lon"]."], ".$atts["zoom"].");
L.tileLayer('https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', {
    attribution: '&copy; <a href=\"https://osm.org/copyright\">OpenStreetMap</a> contributors'
}).addTo(map);
L.marker([".$atts["lat"].", ".$atts["lon"]."]).addTo(map)
    .bindPopup('".$atts["popup"]."').openPopup();};
addOnloadEvent(onload_event_".$_simple_leaflet_demo_map_number.");
</script>";
  return $str;
}

add_shortcode('simple-leaflet-demo','simple_leaflet_demo');
?>