loaddata.js

<script type="text/javascript" src="DataTables/datatables.min.js"></script>
    <script>
    var theTable = null;

    $(document).ready(function() {
        theTable = $('#table-customer').DataTable({
            "processing": true,
            "serverSide": false,
            "ordering": true, 
            "order": [[ 0, 'asc' ]], 
            "ajax":
            {
                "url": "http://localhost/myshop/showdata.php", 
                "type": "GET"
            },
            "sAjaxDataProp": "data",
            "deferRender": true,
            "aLengthMenu": [[5, 10, 20, 50],[ 5, 10, 20, 50]], 
            "columns": [
                { data: "cust_id" }, 
                { data: "cust_name" }, 
                { data: "cust_addr" },
                { data: "cust_email" },
                { data: "cust_phone" },
                { data: "cust_photo", sortable: true, render: function (o) { return '<button id="btn1" onclick="displayImage(data)">Show</button>';}
            ]
        });
    });

    function displayImage(link){
      window.alert(link);
    }

    </script>