If/else statements in PHP and Ajax input validation
if(isset($_POST['username'])  && isset($_POST['email']) && isset($_POST['email2']) && isset($_POST['password'])
&& isset($_POST['firstname']) && isset($_POST['surname']) && isset($_POST['gender']) && isset($_POST['day'])
&& isset($_POST['month'])  && isset($_POST['year']) ) {

    $username = $_POST['username'];
    $email = $_POST['email'];
    $email2 = $_POST['email2'];
    $password = $_POST['password'];
    $firstname = $_POST['firstname'];
    $surname = $_POST['surname'];
    $gender = $_POST['gender'];
    $day = $_POST['day'];
    $month = $_POST['month'];
    $year = $_POST['year'];


    if(!preg_match("/^[a-z](?=[w.]{3,19}$)w*.?w*$/i",$username)){

        echo "not a valid username.";
    }
    else if(filter_var($email,FILTER_VALIDATE_EMAIL)){

        echo "OK!";
    }
    else if(!filter_var($email,FILTER_VALIDATE_EMAIL)){

        echo "not a valid email address";
    }
    else if(strcmp($email,$email2) != 0){

        echo "emails are different.";
    }
    else if(strcmp($email,$email2) == 0){

        echo "OK!";
    }
    else if(!preg_match("[a-zA-Z]*",$firstname)){

        echo "Not a valid firstname.";

    }
    else if(preg_match("[a-zA-Z]*",$firstname)){

        echo "OK!";

    }
    else if(!preg_match("[a-zA-Z]*",$surname)){

        echo "not a valid surname.";

    }
    else if(preg_match("[a-zA-Z]*",$surname)){

        echo "OK!";
    }
}

function handlePost() {

 var username = $('#username').val();
 var email = $('#email').val();
 var email2 = $('#email2').val();
 var password = $('#password').val();
 var firstname = $('#firstname').val();
 var surname = $('#surname').val();
 var gender = $('#gender').val();
 var day = $('#day').val();
 var month = convertMonth($('#month').val())
 var year = $('#year').val();

 $.ajax({
   type: "POST",
   url: "handleRegister.php",
   data: "username="+username+"&email="+email+"&email2="+email2+"&password="+password+"&firstname="
   +firstname+"&surname="+surname+"&gender="+gender+"&day="+day+"&month="+month+"&year="+year,
   success: function(resp){
     // we have the response
     //alert("Server said:n '" + resp + "'");

     console.log("Server said:n '" + resp + "'")
   },
   error: function(e){
     //alert('Error: ' + e);
     console.log("Server said:n '" + e + "'")
   }
 });
}

$errors = array();

if (!preg_match("/^[a-z](?=[w.]{3,19}$)w*.?w*$/i", $username))
{
    $errors[] = "not a valid username.";
}
if (!filter_var($email, FILTER_VALIDATE_EMAIL))
{
    $errors[] = "not a valid email address";
}
if ($email !== $email2)
{
    $errors[] = "emails are different";
}
if (!preg_match("[a-zA-Z]*", $firstname))
{
    $errors[] = "Not a valid firstname.";
}
if (!preg_match("[a-zA-Z]*", $surname))
{
    $errors[] = "not a valid surname.";
}

if ($errors)
{
    echo implode("n", $errors);
}
else
{
    echo 'OK!';
}

if(!preg_match("/^[a-z](?=[w.]{3,19}$)w*.?w*$/i",$username)){

        echo "not a valid username.";
    }
 if(filter_var($email,FILTER_VALIDATE_EMAIL)){

        echo "OK!";
 }
 if(!filter_var($email,FILTER_VALIDATE_EMAIL)){

    echo "not a valid email address";
 }
 if(strcmp($email,$email2) != 0){

     echo "emails are different.";
 }
 etc....