<?php
foreach ($required as $requiredField) {
    if (array_key_exists($requiredField, $_POST)) {
            switch ($requiredField) {

                case 'username':
//                  if (empty($_POST['requiredField'])) {
//                      $error[] = $requiredField . "is required";
//                  } else {
// this is already checked with array_key_exists
                            if (!preg_match("/^[a-zA-Z ]*$/", $_POST[$requiredField])) {
                                // this means if $_POST['username'] doesn't match condition, error will be pushed
                                $error[] = "only letters and white space is allowed";
                            } else {
                                $username = $_POST[$requiredField];
                                // $_POST[$requiredField] would be $_POST['username'] when using switch case
                            }
//                  }
                break;