Meta Box Generator

                    class wpturbo-custom-meta-box{

	private $display_locations = array(
	);
	
	private $fields = array(
	);
	
	public function __construct(){
		add_action( 'add_meta_boxes', array( $this, 'wpturbo_register_meta_boxes' ) );
		add_action( 'save_post', array( $this, 'wpturbo_save_meta_box_fields' ) );
	}
	public function wpturbo_register_meta_boxes(){
		foreach ( $this->display_locations as $location ) {
			add_meta_box(
				'wpturbo-custom-meta-box', // The id of our meta box.
				'My Custom Meta Box', // The title of our meta box.
				array( $this, 'wpturbo_render_meta_box_fields' ), // The callback function that renders the metabox.
				$location, // The screen on which to show the box.
				'normal', // The placement of our meta box.
				'default', // The priority of our meta box.
			);
		}
	}
	// Render the meta box and its fields.
	public function wpturbo_render_meta_box_fields($post){
		wp_nonce_field( 'wpturbo-custom-meta-box_data', 'wpturbo-custom-meta-box_nonce' );
		echo '<h3>My Custom Meta Box</h3>';
		$html = '';
		foreach( $this->fields as $field_id => $field ){
			$meta_value = get_post_meta( $post->ID, $field_id, true );
			if ( empty($meta_value) && isset( $field['default'] ) ) {
				$meta_value = $field['default'];
			}
	
			$field_html = $this->wpturbo_render_input_field($field_id, $field, $meta_value);
			$label = "<label for='$field_id'>{$field['label']}</label>";
			$html .= $this->wpturbo_format_field($label, $field_html);
		}
		echo '<table class="form-table"><tbody>' . $html . '</tbody></table>';
	}
	
	// Format every field to table display.
	public function wpturbo_format_field($label, $field){
		return '<tr class="form-field"><th>'.$label.'</th><td>'.$field.'</td></tr>';
	}
	
	// Render each individual field.
	public function wpturbo_render_input_field($field_id, $field, $field_value){
		switch($field['type']){
			case 'select': {
				$field_html = '<select name="'.$field_id.'" id="'.$field_id.'">';
					foreach($field['options'] as $key => $value){
						$key = !is_numeric( $key ) ? $key : $value;
						$selected = '';
						if($field_value === $key){
							$selected = 'selected="selected"';
						}
						$field_html .= '<option value="'.$key.'" '.$selected.'>'.$value.'</option>';
					}
				$field_html .= '</select>';
				break;
			}
			case 'textarea': {
				$field_html = '<textarea name="'.$field_id.'" id="'.$field_id.'" rows="6">'.$field_value.'</textarea>';
				break;
			}
			default: {
				$field_html = "<input type='{$field['type']}' id='$field_id' name='$field_id' value='$field_value' />";
				break;
			}
		}
	
		return $field_html;
	}
	
	// Called when this metabox is saved.
	// Save the new meta values of our metabox.
	public function wpturbo_save_meta_box_fields($post_id){
		if ( ! isset( $_POST['wpturbo-custom-meta-box_nonce'] ) ) return $post_id;
	
		$nonce = $_POST['wpturbo-custom-meta-box_nonce'];
		if ( !wp_verify_nonce( $nonce, 'wpturbo-custom-meta-box_data' ) ) return $post_id;
	
		if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return $post_id;
	
		foreach ( $this->fields as $field_id => $field ) {
			if( isset($_POST[$field_id]) ){
				// Sanitize fields that need to be sanitized.
				switch($field['type']){
					case 'email': {
						$_POST[$field_id] = sanitize_email($_POST[$field_id]);
						break;
					}
					case 'text': {
						$_POST[$field_id] = sanitize_text_field($_POST[$field_id]);
						break;
					}
				}
				update_post_meta( $post_id, $field_id, $_POST[$field_id] );
			}
		}
	}
	
}

if (class_exists('wpturbo-custom-meta-box')) {
	new wpturbo-custom-meta-box();
}