I'm having problems with validation in javascript

I'm having problems with validation in javascript

I am new to PHP, I had a problem in registering a user in Database. I want
to first validate form using javascript then form action to perform which
will redirect it to other page having PHP code and SQL queries. But
problem is it only validate Username and Email. All other field are not
checked by javascript, please help.
Register.php
<script type="text/javascript">
function initthis()
{
document.getElementById("signup").disabled=true;
}
function dis_enable_submit()
{
if(document.getElementById("t&d").checked == 1)
{
document.getElementById("signup").disabled=false;
document.getElementById("signup").className="enabled";
}
else
{
document.getElementById("signup").disabled=true;
document.getElementById("signup").className="disabled";
}
}
window.onload=initthis;
function validateForm()
{
var a=document.forms["register"]["duser"].value;
var b=document.forms["register"]["demail"].value;
var c=document.forms["register"]["dcemail"].value;
var d=document.forms["register"]["dpwd"].value;
var e=document.forms["register"]["dcpwd"].value;
if (a==null || a=="")
{
document.getElementById("duser").style.background="#FDFCDC";
return false;
}
var atpos=b.indexOf("@");
var dotpos=b.lastIndexOf(".");
if (atpos<1 || dotpos<atpos+2 || dotpos+2>=b.length)
{
document.getElementById("demail").style.background="#FDFCDC";
return false;
}
else{document.getElementById("tick").src="images/tick.gif";}
if(c==null || c=="")
{
document.getElementById("dcemail").style.background="#FDFCDC";
alert("Email doesn't match");
return false;
}
if (c!=b)
{
alert("Email doesn't match");
return false;
}
if(d==null || d=="")
{
document.getElementById("dpwd").style.background="#FDFCDC";
return false;
}
if(e==null || e=="")
{
document.getElementById("dcpwd").style.background="#FDFCDC";
alert("Password doesn't match");
return false;
}
if (e!=d)
{
alert("Password doesn't match");
return false;
}
}
This is FORM which is to be validate
</td>
</tr>
<tr>
<td>
<label>*Username:</label>
</td>
<td>
<input name="duser" id="duser" value="" type="text" spellcheck="false"
size="20" />
</td>
</tr>
<tr>
<td>
<label>*Email:</label>
</td>
<td>
<input name="demail" id="demail" value="" type="text" size="20" />
</td>
</tr>
<tr>
<td>
<label>*Confirm Email:</label>
</td>
<td>
<input name="dcemail" id="dcemail" value="" type="text" size="20" />
</td>
</tr>
<tr>
<td>
<label>*Password:</label>
</td>
<td>
<input name="dpwd" id="dpwd" value="" type="password" size="20" />
</td>
</tr>
<tr>
<td>
<label>*Confirm Password:</label>
</td>
<td>
<input name="dcpwd" id="dcpwd" value="" type="password" size="20" />
</td>
</tr>
<tr>
<td>
<label>*Gender:</label>
</td>
<td>
<input type="radio" name="gender" id="gender" value="1">Male <input
type="radio" name="gender" value="0">Female
</td>
</tr>
<tr>
<td>
<label> Birthday: </label>
</td>
<td>
<input name="bday" id="bday" type="date" size="20" />
</td>
</tr>
<tr>
<td align="middle">
<input id="signup" class="button" type="submit" value="Sign Up"
style="font-size:15px;" />
</td>
</tr>
</table>
</form>
getdata.php
<?php
$_POST[duser];
$_POST[demail];
$_POST[dpwd];
$_POST[gender];
$con=mysqli_connect("localhost","root","root","MyDB");
// Check connection
if (mysqli_connect_errno())
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
mysqli_query($con,"INSERT INTO Users (UserName, Email, Password, Gender)
VALUES ('$_POST[duser]', '$_POST[demail]','$_POST[dpwd]',$_POST[gender] )");
mysqli_close($con);
header("Location: successful.php");
exit();
?>