I would like to better understand a feature commonly used in web systems where you access a page it displays a loader while content is not ready.
How to implement this in a PHP system that does some delivery procedures for the front end that also handles some DOM's?
Boot a <div> with
position: absolute;
top: 0px;
left: 0px;
z-index: 100;
Inside it you put your loader image, just do the style to center it.
It's just logical to make it appear or hide.
Ex:
<div id="meuLoader">...</div>
<script>
// rolou algum processamento no seu .php ou vc fez um ajax
$('#meuLoader').addClass('hidden'); // classe do bootstrap que coloca estilo 'display: none';
</script>