<input type="number" id="numInput" placeholder="請輸入一個數字">
<button onclick="squareNumber()">計算平方</button>

<script>
function squareNumber() {
   var num = document.getElementById("numInput").value;
   var xmlhttp = new XMLHttpRequest();
   xmlhttp.onreadystatechange = function() {
     if (this.readyState == 4 && this.status == 200) {
       document.getElementById("result").innerHTML = this.responseText;
     }
   };
   xmlhttp.open("GET", "squareNumber.php?num=" + num, true);
   xmlhttp.send();
}
</script>

<?php
   $num = $_GET["num"];
   $result = $num * $num;
   echo $result;
?>