if(isset($_POST['submit'])) {
  $author = $_POST['author'];
  $article_body = $_POST['article_body'];
  $article_title = $_POST['article_title'];
  $publication_date = $_POST['publication_date'];
  ...
  $category = $_POST['category'];

  #validation code
  ...
}

foreach($_POST as $key => $value) {
  ${"$key"} = $value;
}

//now I access all of that variables
echo $author; //will work
echo $publication_date; //will also work

$Form = new Form;
$Form->get_all_posted_values()->unset_field(['submit']);

foreach(array_keys($Form->fields) as $key) {
  ${"$key"} = $Form->fields["$key"];
}

$author->required()->type("name")->range(8, 31)->label("Author")->error_message("The author name cannot have numbers or special characters.");

$title->required()->type("string")->range(8, 127)->label("Article title")->error_message("The title must be descriptive and should not be too short or too long");

$category->required()->type("string")->range(3 ,31)->label("Article category")->error_message("Category must be descriptive and must have be at least 3 characters long");

$body->required()->type("html")->range(600, 65535)->label("Article body")->error_message("The article should have at least 600 characters and at the most 65535 characters");

$publication_date->required()->type("date")->min("06/01/2019")->max("01/01/2020")->label("Publication date")->error_message("Set a publication date for this year");

Class Form {
  #code
  ...

  public function create_vars_for_fields() {
    foreach(array_keys($Form->fields) as $key) {
      ${"$key"} = $Form->fields["$key"];
    }
    return $this;
  }
}

require_once 'Form.php';

$Form = new Form;
$Form->get_all_posted_values()->create_vars_for_fields();

//here I should be able to access my variables, which are now objects of the class Fields
echo $author->value; //should echo the name of the author, which is equal to $_POST['author']
echo $title->value; //should echo the article title, which is equal to $_POST['title']