Slash Command

Formerly known as slash commands, DiscordPHP v7+ supports all types of Discord Application Commands

Here is the example of /ping command:

use Discord\Parts\Interactions\Command\Command; // Notice to use this correct namespace!

$discord->on('ready', function (Discord $discord) {
    // When bot is ready, attempt to create a global slash command
    $command = new Command($discord, ['name' => 'ping', 'description' => 'Pong!']);
    $discord->application->commands->save($command);
});

// Handle the command
$discord->listenCommand('ping', function (Interaction $interaction) {
    // Respond the /ping command with interaction message "pong!"
    $interaction->respondWithMessage(MessageBuilder::new()->setContent('pong!'));
});

For more information about Interaction, see the documentation.

Note: In order for your global application command to show up in a guild, you must authorize the Bot using applications.commands scope, the URL can be generated from Discord Developers Portal

And then you can double check the command in Server Settings > Integrations