create-custom-metabox.php

<?php

/**
 * Creates a custom meta box.
 *
 * @param string   $post_type Post type.
 * @param WP_Post $post Post.
 */
function prefix_add_meta_box( $post_type, $post ) {

	// If you want to add the meta box to all post types, remove this line.
	if ( 'my_custom_post_type' !== $post_type ) {
		return;
	}

	add_meta_box(
		'my_custom_meta_box', // ID. Should be unique.
		__( 'Custom Metabox', 'my-text-domain' ), // Title.
		'prefix_meta_box_callback', // Display the meta box in this function.
		'my_screen_id', // The screen id.
		'side', // can be 'normal', 'advanced', or 'side'.
		'high' // can be 'high', 'default' or 'low'.
	);

}
add_action( 'add_meta_boxes', 'prefix_add_meta_box', 10, 2 );

/**
 * Display the meta box.
 *
 * @param WP_Post $post Post.
 */
function prefix_meta_box_callback( $post ) {

	// Add an nonce field so we can check for it later.
	wp_nonce_field( 'my_meta_box_nonce', 'prefix_meta_box_nonce' );

	// Use get_post_meta to retrieve an existing value from the database.
	$value = get_post_meta( $post->ID, '_my_meta_value_key', true );

	// Display the form fields.
	?>
		<p>
			<label for="my_meta_value"><?php _e( 'This is a meta box.', 'my-text-domain' ); ?></label>
			<input type="text" name="my_meta_value" id="my_meta_value" value="<?php echo esc_attr( $value ); ?>" class="regular-text" />
		</p>
	<?php
}

/**
 * Save the meta box.
 *
 * @param int $post_id Post ID.
 * @param  WP_Post $post Post object.
 */
function prefix_meta_box_save( $post_id, $post ) {

	// Check if our nonce is set.
	if ( ! isset( $_POST['prefix_meta_box_nonce'] ) ) {
		return;
	}

	// Verify that the nonce is valid.
	if ( ! wp_verify_nonce( $_POST['prefix_meta_box_nonce'], 'my_meta_box_nonce' ) ) {
		return;
	}

	// Do not save for ajax requests.
	if ( ( defined( 'DOING_AJAX' ) && DOING_AJAX ) || isset( $_REQUEST['bulk_edit'] ) ) {
		return;
	}

	// Dont' save meta boxes for revisions or autosaves.
	if ( ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) || is_int( wp_is_post_revision( $post ) ) || is_int( wp_is_post_autosave( $post ) ) ) {
		return;
	}

	// Check the post being saved == the $post_id to prevent triggering this call for other save_post events.
	if ( empty( $_POST['post_ID'] ) || absint( $_POST['post_ID'] ) !== $post_id ) {
		return;
	}

	// Check user has permission to edit.
	if ( ! current_user_can( 'edit_post', $post_id ) ) {
		return;
	}

	// Sanitize the user input.
	$my_meta_value = sanitize_text_field( $_POST['my_meta_value'] );

	// Update the meta field.
	update_post_meta( $post_id, '_my_meta_value_key', $my_meta_value );

	// Remove the save_post action.
	remove_action( 'save_post', 'prefix_meta_box_save' );
}
add_action( 'save_post', 'prefix_meta_box_save', 10, 2 );