Photo Credit

class dd_slider_meta {

	public function __construct() {

		if ( is_admin() ) {
			add_action( 'load-post.php',     array( $this, 'init_metabox' ) );
			add_action( 'load-post-new.php', array( $this, 'init_metabox' ) );
		}

	}

	public function init_metabox() {

		add_action( 'add_meta_boxes',        array( $this, 'add_metabox' )         );
		add_action( 'save_post',             array( $this, 'save_metabox' ), 10, 2 );

	}

	public function add_metabox() {

		add_meta_box(
			'dd_slider_meta',
			__( 'Slider Extra Data', 'text_domain' ),
			array( $this, 'render_metabox' ),
			'slider',
			'normal',
			'default'
		);

	}

	public function render_metabox( $post ) {

		// Retrieve an existing value from the database.
		$dd_photo_credit = get_post_meta( $post->ID, 'dd_photo_credit', true );

		// Set default values.
		if( empty( $dd_photo_credit ) ) $dd_photo_credit = '';

		// Form fields.
		echo '<table class="form-table">';

		echo '	<tr>';
		echo '		<th><label for="dd_photo_credit" class="dd_photo_credit_label">' . __( 'Photo Credit', 'text_domain' ) . '</label></th>';
		echo '		<td>';
		echo '			<input type="text" id="dd_photo_credit" name="dd_photo_credit" class="dd_photo_credit_field" placeholder="' . esc_attr__( '', 'text_domain' ) . '" value="' . esc_attr( $dd_photo_credit ) . '">';
		echo '		</td>';
		echo '	</tr>';

		echo '</table>';

	}

	public function save_metabox( $post_id, $post ) {

		// Sanitize user input.
		$dd_new_photo_credit = isset( $_POST[ 'dd_photo_credit' ] ) ? sanitize_text_field( $_POST[ 'dd_photo_credit' ] ) : '';

		// Update the meta field in the database.
		update_post_meta( $post_id, 'dd_photo_credit', $dd_new_photo_credit );

	}

}

new dd_slider_meta;