WC_Settings_API::get_field_value PHP Method

    public function get_field_value($key, $field, $post_data = array())
    {
        $type = $this->get_field_type($field);
        $field_key = $this->get_field_key($key);
        $post_data = empty($post_data) ? $_POST : $post_data;
        $value = isset($post_data[$field_key]) ? $post_data[$field_key] : null;
        // Look for a validate_FIELDID_field method for special handling
        if (is_callable(array($this, 'validate_' . $key . '_field'))) {
            return $this->{'validate_' . $key . '_field'}($key, $value);
        }
        // Look for a validate_FIELDTYPE_field method
        if (is_callable(array($this, 'validate_' . $type . '_field'))) {
            return $this->{'validate_' . $type . '_field'}($key, $value);
        }
        // Fallback to text
        return $this->validate_text_field($key, $value);
    }