site_loader.php

<style>
#loader {
  position: absolute;
  left: 50%;
  top: 50%;
  z-index: 1;
  width: 150px;
  height: 150px;
  margin: -75px 0 0 -75px;
  border: 16px solid #f3f3f3;
  border-radius: 50%;
  border-top: 16px solid #3498db;
  width: 120px;
  height: 120px;
  -webkit-animation: spin 2s linear infinite;
  animation: spin 2s linear infinite;
}

@-webkit-keyframes spin {
  0% { -webkit-transform: rotate(0deg); }
  100% { -webkit-transform: rotate(360deg); }
}

@keyframes spin {
  0% { transform: rotate(0deg); }
  100% { transform: rotate(360deg); }
}

</style>
<body onload="myFunction()" style="margin:0;">

<div id="loader"></div>

<div style="display:none;" id="myDiv" class="animate-bottom">
</div>

<script>
var myVar;

function myFunction() {
  myVar = setTimeout(showPage, 3000);
}

function showPage() {
  document.getElementById("loader").style.display = "none";
}
</script>