register.php

<html>
<head>
<title>Registration Form</title>
</head>

<script language="javascript">
    function isEmpty(){
        if (document.forms['input_form'].imsi.value == "" || document.forms['input_form'].msisdn.value == "" || document.forms['input_form'].sdid.value == ""){
            alert("All required values must be inputted.");
            return false;
        }
        else return true;
    }
</script>

<body>

<form name="input_form" action="register.php" method="post">
    <center>
       <br>
        IMSI: <input type="text" name="imsi"><br>
        MSISDN: <input type="text" name="msisdn"><br>
        SDID: <input type="text" name="sdid"><br>
        <br>
    <input type="submit" onClick="return isEmpty();" value="Register">
</center>
</form>

</body>
</html>