DIsplay msg if radiobutton is not selected by user

<html>
<title>Micro Elite Brigade - Registration</title><LINK REL="SHORTCUT ICON" HREF="images/favicon.png"><?php	
require_once('upper.php');
require_once('database.php');
echo $error_msg='';
if(isset($_POST['submit']))
{
$LoginId=mysqli_real_escape_string($dbc,trim($_POST['LoginId']));
$Password1=mysqli_real_escape_string($dbc,trim($_POST['Password1']));
$Password2=mysqli_real_escape_string($dbc,trim($_POST['Password2']));
$Name=mysqli_real_escape_string($dbc,trim($_POST['Name']));
$Age=mysqli_real_escape_string($dbc,trim($_POST['Age']));
$BloodGroup=mysqli_real_escape_string($dbc,trim($_POST['BloodGroup']));
/*if(!isset($_POST['Sex']))
{
echo 'Please enter Sex<br>';
}*/
//else{
$Sex= mysqli_real_escape_string($dbc,trim($_POST['Sex']));
//}
$Qualification=mysqli_real_escape_string($dbc,trim($_POST['Qualification']));
$ContactNumber=mysqli_real_escape_string($dbc,trim($_POST['ContactNumber']));
$Email=mysqli_real_escape_string($dbc,trim($_POST['Email']));
$Address=mysqli_real_escape_string($dbc,trim($_POST['Address']));
$AboutYourself=mysqli_real_escape_string($dbc,trim($_POST['AboutYourself']));
//$countCheck=count($_POST['checkbox']);
//echo $countCheck;
//$checkbox=$_POST['checkbox'];
//$countCheck=count($checkbox);
	if(empty($LoginId)){echo 'Please enter Login Id';}
	elseif(empty($Password1)){echo 'Please enter Password';}
	elseif(empty($Password2)){echo 'Please confirm Password';}
	elseif($Password1!==$Password2){echo 'Password didn\\'t match';}
		elseif(empty($Name)){echo 'Please enter Name';}
		elseif(empty($Age)){echo 'Please enter Age';}
		elseif(!isset($_POST['Sex'])){echo 'Please enter Sex';}
		elseif(empty($Qualification)){echo 'Please enter Qualification';}
		elseif(empty($ContactNumber)){echo 'Please enter Contact Number';}
		elseif(empty($Email)){echo 'Please enter Email';}
		elseif(empty($Address)){echo 'Please enter Address';}
		elseif(empty($AboutYourself)){echo 'Please enter About Yourself';}
		elseif(!isset($_POST['checkbox'])){ echo 'You have to register at least one activity.';}
		elseif(!isset($_POST['TermsAndConditions'])){ echo 'You have to agree all Terms and Conditions of Elite Brigade.';}
		else
		{
		require_once('database.php');
		$query="select * from registration where LoginId='$LoginId'";
		$result=mysqli_query($dbc,$query);
		if(mysqli_num_rows($result)==0)	
		{			
		$checkbox=$_POST['checkbox'];
		$countCheck=count($_POST['checkbox']);
		$reg_id=' ';
		for($i=0;$i<$countCheck;$i++)
		{
		$reg_id=$reg_id.$checkbox[$i].',';
		$query="insert into activity_participation (LoginId,Title,Date) values ('$LoginId','$checkbox[$i]',CURDATE())";
		$result=mysqli_query($dbc,$query) or die("Not Connected");
		}
		$query="insert into registration (LoginId,Password,Name,Age,BloodGroup,Sex,Qualification,ContactNumber,Email,Address,AboutYourself,Activity)values ('$LoginId',SHA('$Password1'),'$Name','$Age','$BloodGroup','$Sex','$Qualification','$ContactNumber','$Email','$Address','$AboutYourself',',$reg_id')";
		$result=mysqli_query($dbc,$query) or die("Not Connect");
		
		echo ' Dear '.$Name.'.<br>Your request has been mailed to admin.<br>Your account is waiting for approval<br>';
		$from= 'Elite Brigade';
		$to='ankitp@rsquareonline.com';
		$subject='New User Registration';
		$message="Dear admin,\
\
A new user request for registration. Please check it out.\
\
Regards\
Micro";
		mail($to,$subject,$message,'From:'.$from);
		//header('Location: index.php');
		//	header('Location: Registration.php');
		}
		else
		{
		echo 'Dear '.$Name. ', <br> An account already exist with login-id<b> '.$LoginId.'</b> <br>Please try another login-id';
		}}	
}		
?>

<html>
<head>
<link rel="stylesheet" type="text/css" href="css/style.css" />
<script type="text/javascript">
function lengthRestriction(elem, min, max){
	var uInput = elem.value;
	if(uInput.length >= min && uInput.length <= max){
		return true;
	}else{
		alert("Please enter between " +min+ " and " +max+ " characters");
		elem.value="";
		return false;
	}
}
function emailValidator(elem, helperMsg){
	var emailExp = /^[\\w\\-\\.\\+]+\\@[a-zA-Z0-9\\.\\-]+\\.[a-zA-z0-9]{2,4}$/;
	if(elem.value.match(emailExp)){
		return true;
	}else{
		alert(helperMsg);
		elem.value="";
		return false;
	}
}
</script>
  
</head>

<body>

<?php
echo $error_msg; ?>

<form action='<?php echo $_SERVER['PHP_SELF'];?>' id="commentForm" method='post'>
<div class="registration_and_activity">

<table border="0" cellspacing="0" cellpadding="0" width="380">
<tr><td colspan="2">
<br/><h3>New User?</h3></td></tr>
<tr><td width="120">

<em>*</em>Enter Login id</td><td width="150"><input type='text' name='LoginId' id='LoginId'  value='<?php if(!empty($LoginId))echo $LoginId;?>' onblur="lengthRestriction(document.getElementById('LoginId'), 6, 20)")/></td></tr>
<tr><td>
<em>*</em>Enter Password</td> <td><head>
      <SCRIPT language=Javascript>

function capLock(e){
 kc = e.keyCode?e.keyCode:e.which;
 sk = e.shiftKey?e.shiftKey:((kc == 16)?true:false);
 if(((kc >= 65 && kc <= 90) && !sk)||((kc >= 97 && kc <= 122) && sk))
 {
 if(document.getElementById('Password1').value=='')
  alert("Caps Lock is On");
  }
 else
  document.getElementById('divMayus').style.visibility = 'hidden';
}

</SCRIPT>

   </HEAD>
<input onkeypress='return capLock(event)' type='password' name='Password1' id="Password1"  value='<?php if(!empty($Password1))echo $Password1;?>' onblur="lengthRestriction(document.getElementById('Password1'), 4, 50)")/></td></tr>

<tr><td>
<em>*</em>Confirm Password</td><td><input type='password' name='Password2' value='<?php if(!empty($Password2))echo $Password2;?>' /></td></tr>
<tr><td width="120">
<em>*</em>Enter Name</td> <td><input type='text'  name='Name' Id="Name" value='<?php if(!empty($Name))echo $Name;?>' onblur="lengthRestriction(document.getElementById('Name'), 2, 30)")/></td></tr>
<tr><td>
<em>*</em>Enter Age</td><HEAD>
   <SCRIPT language=Javascript>
      
      function isNumberKey(evt)
      {
         var charCode = (evt.which) ? evt.which : event.keyCode
         if (charCode > 31 && (charCode < 48 || charCode > 57))
            return false;

         return true;
      }
	   </SCRIPT>
   </HEAD>
<td><INPUT onkeypress='return isNumberKey(event)'  type='text' name='Age' value='<?php if(!empty($Age))echo $Age;?>'/></td></tr>

<tr><td>
<em>*</em>Enter Blood</td><td><input type='text' name='BloodGroup' value='<?php if(!empty($BloodGroup))echo $BloodGroup;?>' id="BloodGroup" onblur="lengthRestriction(document.getElementById('BloodGroup'), 1, 3)") /></td></tr>

<tr><td>
<em>*</em>Enter Sex</td><td><input type='radio' name='Sex'  style='width:16px; border:0;' value='Male'<?php if(isset($_POST['Sex'])) { echo "checked='checked'";} ?> />Male   <input type='radio' name='Sex' style='width:16px; border:0;' value='Female' <?php if(isset($_POST['Sex'])) { echo "checked='checked'";} ?> />Female</td></tr>

<tr><td>
<em>*</em>Enter Qualification</td><td><input type='text' name='Qualification'  value='<?php if(!empty($Qualification))echo $Qualification;?>' id="Qualification" onblur="lengthRestriction(document.getElementById('Qualification'), 3, 60)"/></td></tr>

<tr><td>
<em>*</em>Contact Number </td><td><input onkeypress='return isNumberKey(event)'type='text'  name='ContactNumber' value='<?php if(!empty($ContactNumber))echo $ContactNumber;?>' /></td></tr>

<tr><td>
<em>*</em>Enter Email</td><td><input type='text' name='Email'class="email" value='<?php if(!empty($Email))echo $Email;?>' id="emailer" onblur="emailValidator(document.getElementById('emailer'), 'Not a Valid Email')"/></td></tr>

<tr><td>
<em>*</em>Enter Address</td><td><input type='text' name='Address' value='<?php if(!empty($Address))echo $Address;?>' id="Address" onblur="lengthRestriction(document.getElementById('Address'), 2, 100)")/></td></tr>


<tr ><td >
<em>*</em>About Yourself </td></tr>
<tr><td colspan="2"><textarea rows='10' cols='40'  name='AboutYourself' id="AboutYourself" onblur="lengthRestriction(document.getElementById('AboutYourself'), 5, 500)") /><?php if(!empty($Address))echo $Address;?></textarea></td></tr>
<tr><td>

<?php echo"
<tr><td colspan='2'><em>*</em><b>Select fields for which you want to register</b></td></tr>";

require_once('database.php');
$query="select * from activity";
$result=mysqli_query($dbc,$query);
while($row=mysqli_fetch_array($result)){
$Title=$row['Title'];
$ActivityId=$row['ActivityId'];
echo "<tr><td>$Title</td>";
echo "<td><input type='checkbox' name='checkbox[]' value='$Title' style='width:14px; text-align:right;'/></td></tr>";//value=$ActivityId tells ActivityId variable extracts with name="checkbox"
echo "<br/>";
}
echo "
<tr>
<table border='0' cellspacing='0' cellpadding='0' width='400' style='margin:10px 0 0 0;'>
<td align='left' valign='top' scope='col' width='80'><em>*</em><input type='checkbox' name='TermsAndConditions'  style='width:14px; text-align:right;'/></td>
<td align='left' valign='top' scope='col'> I agree all <a href='TermsAndConditions.php'>Terms and conditions </a>of Elite Brigade.</td>
</table>
</tr>";
echo "<tr><td colspan='2' align='center'><input type='submit' value='Register' name='submit' style='background:url(./images/button_img2.png) no-repeat 10px 0px; width:100px; padding:3px 0 10px 0; color:#FEFBC4; border:0; margin:15px 0 5px 100px; '/></td></tr><br>";
	
echo " </td></tr></table>

</div>

</form>
</body>
</html>";
require_once('lower.php');

?>

Hi friends…
I have two problems with this page…
1–> When user not select radio button it should displays “Please enter Sex” but on submit it displays an error “Undefined index: Sex in C:\wamp\www\EliteBrigadeserver\RegistrationAndActivity.php on line 19
Please enter Sex.”
I want to remove this notice…

2–> If user not fill any field and press submit then if user once selected his sex, it should remain selected.

Help me plz…
Anyone???
Thx in advance…

Line 19 is:

$Sex= mysqli_real_escape_string($dbc,trim($_POST['Sex']));

Because, no value is checked at this point, or maybe because the form was not submitted yet, $_POST[‘Sex’] is not defined.
You have to check it using isset first.