този код работи добре за вас...
$con=mysqli_connect("localhost","root","","my_db");
$check="SELECT * FROM persons WHERE Email = '$_POST[eMailTxt]'";
$rs = mysqli_query($con,$check);
$data = mysqli_fetch_array($rs, MYSQLI_NUM);
if($data[0] > 1) {
echo "User Already in Exists<br/>";
}
else
{
$newUser="INSERT INTO persons(Email,FirstName,LastName,PassWord) values('$_POST[eMailTxt]','$_POST[NameTxt]','$_POST[LnameTxt]','$_POST[passWordTxt]')";
if (mysqli_query($con,$newUser))
{
echo "You are now registered<br/>";
}
else
{
echo "Error adding user in database<br/>";
}
}