Q:

How to merge two arrays

I've got a form and I use it to update a user profile. The problem is that I don't want to make all the fields manadatory so if the user leaves a blank field then the value already stored in the database gets updated with a blank or null value. Therefore I've decided to query the database and get an array of values already stored, then I've got an array of values from the user form. I need to merge the two arrays in a way that I can update the database field if a new value has been inserted in the form and keep the old database value if the form field is blank. 
I know I could do something like the following withut using arrays but I don't know how to use prepared statement in this case
if(!empty($user_last))
  $update_values[] = "user_last='".$user_last."'";
$update_values_imploded = implode(', ', $update_values);

if( !empty($update_values) ){
 $q = "UPDATE users SET $update_values_imploded WHERE user_id='$userid' ";
 $r = mysqli_query($conn,$q);

 if($r){
$_SESSION['success_msg'] = 'profile updated!';
header("location: ../client_profile.php");
exit();
}
}

The array method I've tryed is the following, but it doesn't really work properly.
$merged_array =  array_unique(array_merge($database_rows, $form_data));

Do you have any other ideas? Many thanks 
This is the full code with a from array simulated
$km_user_id= 2;

// this array comes from the form
$from_array = array(
    'km_user_first_name' => 'Antonio', 
    'km_user_last_name' => 'Acri',
    'km_user_address' => 'via pola',
    'km_user_city' => 'roma',
    'km_user_city_prov' => '',
    'km_user_postcode' => '',
    'km_user_email' => '',
    'km_user_website' => 'url',
    'km_user_telephone' =>  '123456',
    'km_user_mobile' => '',
    'km_user_fiscalcode' => '',
    'km_user_document' => '',
    'km_user_document_number' => '',
    'km_user_document_exp' =>  '',
    'km_user_birth_place' => '',
    'km_user_birth_date' => ''

);

   // select vakues from database

   $query= "SELECT km_user_first_name, km_user_last_name, km_user_address, km_user_city, km_user_city_prov, km_user_postcode, km_user_email, km_user_website, km_user_telephone, km_user_mobile, km_user_fiscalcode, km_user_document, km_user_document_number, km_user_document_exp, km_user_birth_place, km_user_birth_date FROM km_users WHERE km_user_id= ?";
   $stmt = mysqli_prepare($db_user_conn, $query);
   mysqli_stmt_bind_param($stmt, 'i', $km_user_id);
   mysqli_stmt_execute($stmt);
   $result = mysqli_stmt_get_result($stmt);

   $row = mysqli_fetch_array($result, MYSQLI_ASSOC);
   print_r($row);

?>
<br>
<br>
<?php

print_r($from_array);

?>
<br>
<br>
<?php

$result =  array_merge($from_array, $row); 
print_r($result);

You can see the value [km_user_address] => via roma 11 supposed to be [km_user_address] => via pola in the merged array.

A:

I want to provide a second answer with a total different approach.
The probably best and most secure way you can do it is just to execute a simple static prepared statement and let the handling of empty parameters up to SQL:
$sql_update = <<<_SQL_

  UPDATE 
    `users`
  SET
    `km_user_first_name`      = COALESCE(NULLIF(?, ''), `km_user_first_name`      ),
    `km_user_last_name`       = COALESCE(NULLIF(?, ''), `km_user_last_name`       ),
    `km_user_address`         = COALESCE(NULLIF(?, ''), `km_user_address`         ),
    `km_user_city`            = COALESCE(NULLIF(?, ''), `km_user_city`            ),
    `km_user_city_prov`       = COALESCE(NULLIF(?, ''), `km_user_city_prov`       ),
    `km_user_postcode`        = COALESCE(NULLIF(?, ''), `km_user_postcode`        ),
    `km_user_email`           = COALESCE(NULLIF(?, ''), `km_user_email`           ),
    `km_user_website`         = COALESCE(NULLIF(?, ''), `km_user_website`         ),
    `km_user_telephone`       = COALESCE(NULLIF(?, ''), `km_user_telephone`       ),
    `km_user_mobile`          = COALESCE(NULLIF(?, ''), `km_user_mobile`          ),
    `km_user_fiscalcode`      = COALESCE(NULLIF(?, ''), `km_user_fiscalcode`      ),
    `km_user_document`        = COALESCE(NULLIF(?, ''), `km_user_document`        ),
    `km_user_document_number` = COALESCE(NULLIF(?, ''), `km_user_document_number` ),
    `km_user_document_exp`    = COALESCE(NULLIF(?, ''), `km_user_document_exp`    ),
    `km_user_birth_place`     = COALESCE(NULLIF(?, ''), `km_user_birth_place`     ),
    `km_user_birth_date`      = COALESCE(NULLIF(?, ''), `km_user_birth_date`      )
  WHERE
    `user_id` = ?
  ;

_SQL_;

$stmt = $db_user_conn->prepare($sql_update);

mysqli_stmt_bind_param
(
  $stmt,  'ssssssssssssssssi',

  $form_data['km_user_first_name'],
  $form_data['km_user_last_name'],
  $form_data['km_user_address'],
  $form_data['km_user_city'],
  $form_data['km_user_city_prov'],
  $form_data['km_user_postcode'],
  $form_data['km_user_email'],
  $form_data['km_user_website'],
  $form_data['km_user_telephone'],
  $form_data['km_user_mobile'],
  $form_data['km_user_fiscalcode'],
  $form_data['km_user_document'],
  $form_data['km_user_document_number'],
  $form_data['km_user_document_exp'],
  $form_data['km_user_birth_place'],
  $form_data['km_user_birth_date'],
  $km_user_id
);

mysqli_stmt_execute($stmt);

