Create a metabox with a single field

metabox-single.php

<?php

/**
 * Create a metabox with a single field.
 * Replace `_namespace` with some namespace for your project to avoid conflicts with other items
 */

	//
	// Create Metabox
	//

	/**
	 * Create the metabox
	 * @link https://developer.wordpress.org/reference/functions/add_meta_box/
	 */
	function _namespace_create_metabox() {
		
		// Can only be used on a single post type (ie. page or post or a custom post type).
		// Must be repeated for each post type you want the metabox to appear on.
		add_meta_box(
			'_namespace_metabox', // Metabox ID
			'Some Metabox', // Title to display
			'_namespace_render_metabox', // Function to call that contains the metabox content
			'post', // Post type to display metabox on
			'normal', // Where to put it (normal = main colum, side = sidebar, etc.)
			'default' // Priority relative to other metaboxes
		);
		
		// To add the metabox to a page, too, you'd repeat it, changing the location
		add_meta_box( '_namespace_metabox', 'Some Metabox', '_namespace_render_metabox', 'page', 'normal', 'default' // Priority relative to other metaboxes );

	}
	add_action( 'add_meta_boxes', '_namespace_create_metabox' );



	/**
	 * Render the metabox markup
	 * This is the function called in `_namespace_create_metabox()`
	 */
	function _namespace_render_metabox() {

		// Variables
		global $post; // Get the current post data
		$details = get_post_meta( $post->ID, '_namespace', true ); // Get the saved values

		?>

			<fieldset>
				<div>
					<label for="_namespace_custom_metabox">
						<?php
							// This runs the text through a translation and echoes it (for internationalization)
							_e( 'Item Name', '_namespace' );
						?>
					</label>
					<?php
						// The `esc_attr()` function here escapes the data for
						// HTML attribute use to avoid unexpected issues
					?>
					<input
						type="text"
						name="_namespace_custom_metabox"
						id="_namespace_custom_metabox"
						value="<?php echo esc_attr( $details ); ?>"
					>
				</div>
			</fieldset>

		<?php

		// Security field
		// This validates that submission came from the
		// actual dashboard and not the front end or
		// a remote server.
		wp_nonce_field( '_namespace_form_metabox_nonce', '_namespace_form_metabox_process' );

	}



	//
	// Save our data
	//

	/**
	 * Save the metabox
	 * @param  Number $post_id The post ID
	 * @param  Array  $post    The post data
	 */
	function _namespace_save_metabox( $post_id, $post ) {

		// Verify that our security field exists. If not, bail.
		if ( !isset( $_POST['_namespace_form_metabox_process'] ) ) return;

		// Verify data came from edit/dashboard screen
		if ( !wp_verify_nonce( $_POST['_namespace_form_metabox_process'], '_namespace_form_metabox_nonce' ) ) {
			return $post->ID;
		}

		// Verify user has permission to edit post
		if ( !current_user_can( 'edit_post', $post->ID )) {
			return $post->ID;
		}

		// Check that our custom fields are being passed along
		// This is the `name` value array. We can grab all
		// of the fields and their values at once.
		if ( !isset( $_POST['_namespace_custom_metabox'] ) ) {
			return $post->ID;
		}

		/**
		 * Sanitize the submitted data
		 * This keeps malicious code out of our database.
		 * `wp_filter_post_kses` strips our dangerous server values
		 * and allows through anything you can include a post.
		 */
		$sanitized = wp_filter_post_kses( $_POST['_namespace_custom_metabox'] );

		// Save our submissions to the database
		update_post_meta( $post->ID, '_namespace', $sanitized );

	}
	add_action( 'save_post', '_namespace_save_metabox', 1, 2 );



	//
	// Save a copy to our revision history
	// This is optional, and potentially undesireable for certain data types.
	// Restoring a a post to an old version will also update the metabox.
	//

	/**
	 * Save events data to revisions
	 * @param  Number $post_id The post ID
	 */
	function _namespace_save_revisions( $post_id ) {

		// Check if it's a revision
		$parent_id = wp_is_post_revision( $post_id );

		// If is revision
		if ( $parent_id ) {

			// Get the saved data
			$parent = get_post( $parent_id );
			$details = get_post_meta( $parent->ID, '_namespace', true );

			// If data exists and is an array, add to revision
			if ( !empty( $details ) ) {
				add_metadata( 'post', $post_id, '_namespace', $details );
			}

		}

	}
	add_action( 'save_post', '_namespace_save_revisions' );



	/**
	 * Restore events data with post revisions
	 * @param  Number $post_id     The post ID
	 * @param  Number $revision_id The revision ID
	 */
	function _namespace_restore_revisions( $post_id, $revision_id ) {

		// Variables
		$post = get_post( $post_id ); // The post
		$revision = get_post( $revision_id ); // The revision
		$details = get_metadata( 'post', $revision->ID, '_namespace', true ); // The historic version

		// Replace our saved data with the old version
		update_post_meta( $post_id, '_namespace', $details );

	}
	add_action( 'wp_restore_post_revision', '_namespace_restore_revisions', 10, 2 );



	/**
	 * Get the data to display on the revisions page
	 * @param  Array $fields The fields
	 * @return Array The fields
	 */
	function _namespace_get_revisions_fields( $fields ) {
		// Set a title
		$fields['_namespace'] = 'Some Item';
		return $fields;
	}
	add_filter( '_wp_post_revision_fields', '_namespace_get_revisions_fields' );



	/**
	 * Display the data on the revisions page
	 * @param  String|Array $value The field value
	 * @param  Array        $field The field
	 */
	function _namespace_display_revisions_fields( $value, $field ) {
		global $revision;
		return get_metadata( 'post', $revision->ID, $field, true );
	}
	add_filter( '_wp_post_revision_field_my_meta', '_namespace_display_revisions_fields', 10, 2 );

Thanks! One thing though: there's a ); missing on line 31