<?php

if(empty($_POST["emailaddress"])){
    $error = 'true';
    $validation_msg = 'Country missing.';
    if(empty($error_msg)){
        $error_msg .= $validation_msg;
    } else{
        $error_msg .= 'n' . $validation_msg;
    }
}

if(empty($_POST["password"])){
    $error = 'true';
    $validation_msg = 'Country missing.';
    if(empty($error_msg)){
        $error_msg .= $validation_msg;
    } else{
        $error_msg .= 'n' . $validation_msg;
    }
}

if(empty($_POST["firstname"])){
    $error = 'true';
    $validation_msg = 'First name missing.';
    if(empty($error_msg)){
        $error_msg .= $validation_msg;
    } else{
        $error_msg .= 'n' . $validation_msg;
    }
}

if(empty($_POST["lastname"])){
    $error = 'true';
    $validation_msg = 'Last name missing.';
    if(empty($error_msg)){
        $error_msg .= $validation_msg;
    } else{
        $error_msg .= 'n' . $validation_msg;
    }
}

if($error){
    header('HTTP/1.1 500 Internal Server Error');
    header('Content-Type: application/json');
    die($error_msg);
}

?>

$error_msg = '';
foreach($_POST as $key => $val){
    if(empty($val)){
        $error = 'true';
        $error_msg .= $key." missing.n";
    }
}

$error_msg = array()

if(empty($_POST["lastname"])){
    $error_msg[] = 'Last name missing.';
}

....

if($error_msg){
    header('HTTP/1.1 500 Internal Server Error');
    header('Content-Type: application/json');
    die(implode("n", $error_msg);
}

$error_msg = array();

$error_msg[] = $validation_msg;

if(count($error_msg) > 0){

<?php
$array = new array{{"firstname","First Name"},{"lastname", "Last Name"}};

if(empty($_POST[$array[i][0])){               $error = 'true';           $validation_msg .= $array[i][1] ' missing.' . "n";

$msgStack = array();

function validate($field, $msg, $msgStack) {
    if(empty($field)) {
        $error = 'true';
        $msgStack[] = $msg;
    }
}

validate($_POST['firstname'], 'Firstname is empty', $msgStack);

echo implode(PHP_EOL, $msgStack);

if(empty($_POST["emailaddress"]) || empty($_POST["password"]) ||
   empty($_POST["firstname"]) || empty($_POST["lastname"]) ){
    $error = TRUE;
    if ( empty($_POST["emailaddress"]) )
      $field = 'Email Address';
   else  if ( empty($_POST["password"]) )
      $field = 'Password';
   else  if ( empty($_POST["firstname"]) )
      $field = 'First Name';
   else
      $field = 'Last Name';

    $validation_msg = $field . ' missing.';
    if(empty($error_msg)){
    $error_msg .= $validation_msg;
    } else{
    $error_msg .= 'n' . $validation_msg;
    }
}