Validation for registration page in PHP

Question

I have a registration page and I want to validate it. I have this code:

$msg = "";
$msg_3 = "";
if(isset($_POST['submit'])) {
  $First_Name = ((isset($_POST['First_Name']))?sanitize($_POST['First_Name']):'');
  $Last_Name = ((isset($_POST['Last_Name']))?sanitize($_POST['Last_Name']):'');
  $email = ((isset($_POST['email']))?sanitize($_POST['email']):'');
  $confirm_email = ((isset($_POST['confirm_email']))?sanitize($_POST['confirm_email']):'');
  $mobile_number = ((isset($_POST['mobile_number']))?sanitize($_POST['mobile_number']):'');
  $password = ((isset($_POST['password']))?sanitize($_POST['password']):'');
  $confirm_password = ((isset($_POST['confirm_password']))?sanitize($_POST['confirm_password']):'');
  $gender = ((isset($_POST['gender']))?sanitize($_POST['gender']):'');
  $day = ((isset($_POST['day']))?sanitize($_POST['day']):'');
  $month = ((isset($_POST['month']))?sanitize($_POST['month']):'');
  $year = ((isset($_POST['year']))?sanitize($_POST['year']):'');
  $insurance = ((isset($_POST['insurance']))?sanitize($_POST['insurance']):'');
  $agree = ((isset($_POST['agree']))?sanitize($_POST['agree']):'');
  $sql = "SELECT email, mobile_number FROM customers WHERE email ='$email' OR mobile_number ='$mobile_number'";
  $result = $db->query($sql);
  if ($result->num_rows > 0) {
    while ($row = $result->fetch_assoc()) {
      if ($email == $row['email']) {
        $msg = "<span class='text-danger'>The email address you've entered is already associated with another account.
        <br>Please sign in or enter a different email address. Please try again.</span>";
      }  if ($mobile_number == $row['mobile_number']) {
        $msg_3 = "<span class='text-danger'>The mobile phone number you've entered is already associated with another account.
        <br>Please sign in or enter a different number. Please try <br>again.</span>";
      }
    }
  } else {
// Insert into database and send email
}

Now how could I validate each field if it is empty and print different messages under each field in this nested if and while. I'm getting confused.

Answer 1

If you will use same names in db as in form you could use something like this:

$keys = ['gender', 'email', 'mobile_number']; //etc

$errors = [];

while ($row = $result->fetch_assoc()) {
    array_walk($keys, function ($key) {
        if (empty($row[$key])) {
            $errors[] = "$key is required"
        }

        if (isset($_POST[$key]) && $_POST[$key] == $row[$key]) {
            $errors[] = "please enter $key"
        }
    })
}

if you need to have more customized messages you might map keys to error text like:

$keys = ['gender' => ['equal' => 'your error message', 'empty' => 'empty msg'], 'email' => ['equal' => 'email validation error', 'empty' => 'error msg 2']]; //etc
$errors = [];

while ($row = $result->fetch_assoc()) {
    array_walk($keys, function ($errorMsg, $key) {

        if (isset($_POST[$key]) && $_POST[$key] == $row[$key]) {
            $errors[$key] = $errorMsg['equal'];
        }

        if (empty($row[$key])) {
            $errors[$key] = $errorMsq['empty'];
        }
    })
}

Answer 2

1. Do not repeat
2. Prevent SQL Injection

You can do something like this.

<?php
if(isset($_POST['submit'])) {

  $errors = [];

  function getPost($postIndex, $errorMessage = '') {
    global $errors;
    if (!empty( $_POST[$postIndex] )) {
      $value = $_POST[$postIndex];
      return $value;;
    } else {
      $errors[$postIndex] = $errorMessage;
      return null;
    }
  }

  function validateString($s) {
    return htmlspecialchars(trim($s));
  }

  getPost('First_Name', 'Firstname Cannot Be Empty');
  getPost('Last_Name', 'Lastname cannot be empty');
  $email = getPost('email', 'Your Error Message');
  getPost('confirm_email', 'Your Error Message');
  $mobile_number = getPost('mobile_number', 'Your Error Message');
  getPost('password', 'Your Error Message');
  getPost('confirm_password', 'Your Error Message');
  getPost('gender', 'Your Error Message');
  getPost('day', 'Your Error Message');
  getPost('month', 'Your Error Message');
  getPost('year', 'Your Error Message');
  getPost('insurance', 'Your Error Message');
  getPost('agree', 'Your Error Message');

  $stmt = $mysqli -> prepare('SELECT email, mobile_number FROM customers WHERE email =? OR mobile_number =?');

  if (
    $stmt &&
    $stmt -> bind_param('ss', $email, $mobile_number) &&
    $stmt -> execute() &&
    $stmt -> store_result() &&
    $stmt -> bind_result($dbEmail, $dbMobileNumber) &&
    $stmt -> fetch()
  ) {

    if ($email == $dbEmail) {
      // email equal error message
    }  if ($mobile_number == $row['mobile_number']) {
      // mobile number equal error message
    }

  }

  if (count($errors)) {
    echo "You have an error";    
  }
  // or get the post index in your HTML form and show the error message there
  // <?php isset($errors['firstName']) ? echo $errors['firstname'] : null; 

}