generic_validation.php

<?php
class Model extends Database
{
  protected function validate_empty($data, $field, $err_message = null)
  {
    if (empty($data[$field])) {
      // Use first part of field name in string  e.g. currency_id required => currency required
      if (empty($err_message)) {
        $err_message = explode("_", $field)[0] . ' required';
      }
      $this->errors[$field] = $err_message;
      return true;
    }
    return false;
  }

  protected function validate_text($data, $field, $regex, $err_msg)
  {
    if (!preg_match($regex, trim($data[$field]))) {
      $this->errors[$field] = $err_msg;
    }
  }
}


class Course_model extends Model
{
 public function validate($data)
  {
    $this->errors = [];

    $this->validate_empty($data, 'title', 'Enter course title')
      || $this->validate_text($data, 'title', "/^[a-zA-Z_ ]+$/", "Course titles can only have letters, spaces and '_'");

    $this->validate_empty($data, 'primary_subject', "Enter a primary subject")
      || $this->validate_text($data, 'title', "/^[a-zA-Z_ ]+$/", "Primary subjects can only have letters, spaces and '_'");

    if (empty($this->errors)) {
      return true;
    }
    return false;
  }
}

?>

Just created some validation functions that were more generic and put them in the Model class, so they can be used by any model. In my opinion, the text validation can be made more generic by grouping common validations. Many of them will be probably be similar.
What do you think?