<?php
$error=array();

function register($username,$password,$confirm_password,$email,$birth){
    global $connection;
    global $error ;
    $add=$connection->query("select username,email from user where username = '$username' OR email = '$email' ");
    $check= $add->FETCH(PDO::FETCH_ASSOC);

    if(isset($_POST['register'])){
        if($username == $check['username']){
            $error['username'] ="this username already exists";
        }elseif($username == $check['email']){
            $error['email'] = "this username already exists";
            print_r($error);
        }else{
            $add=$connection->prepare("insert into user (id,username,password,email,birth) values(NULL,:username,:password,:email,:birth) ");
            $add->execute(array(':username'=>$username,':password'=>$password ,':email'=>$email,':birth'=>$birth));
            if(isset($add)){
                echo ' has been registerd ';
            }
        }
    }

    return $error;
}

function error_function($error=array()){
    foreach($error as $key => $value ){
        echo "$value";
    }
}
error_function($error);
?>