Awesome

Discord SDK for PHP

An ultra-lightweight PHP SDK for accessing the Discord API and Discord webhook endpoints. This SDK does not currently support the Discord Gateway (i.e. the WebSocket interface).

Features

Does what it says on the tin in a mere 4KB of code.
Works with Discord and compatible services.
Has a liberal open source license. MIT or LGPL, your choice.
Designed for relatively painless integration into your project.
Sits on GitHub for all of that pull request and issue tracker goodness to easily submit changes and ideas respectively.

Getting Started (API)

Set up Discord API access:

Visit https://discord.com/developers/applications.
Create a New Application. Name it something sensible such as "Invite Bot" or whatever the associated bot will do in the server.
Go to the Bot tab and enable the bot user.
Turn off the "Public Bot" and "Requires OAuth2 Code Grant" options under the Authorization Flow section.
Scroll to the bottom of the Bot section and calculate the permissions integer for the APIs you plan on accessing. The example code below requires the "Create Instant Invite" permission.
Go to the General Information tab and copy the "Client ID" to the clipboard.
In a new browser tab, visit https://discord.com/oauth2/authorize?client_id=YOUR_APP_CLIENT_ID&scope=bot&permissions=YOUR_PERMISSIONS_INTEGER
If all goes well, Discord should ask which server the bot should join and then perform a verification step.
Back in the Application, go to the Bot tab and copy the "Token" to the clipboard.

Create a 30 minute, single use invite for temporary members:

<?php
	require_once "support/sdk_discord.php";

	// Replace the string with the bot token above.
	$bottoken = "YOUR_BOT_TOKEN_HERE";

	// Replace this with a valid channel ID.
	// The channel ID can be found at the end of the URL in Discord.
	$channelid = "YOUR_CHANNEL_ID_HERE";

	// Create a temporary invite.
	$discord = new DiscordSDK();
	$discord->SetAccessInfo("Bot", $bottoken);

	// For a complete list of options:
	// https://discord.com/developers/docs/resources/channel#create-channel-invite
	$options = array(
		"max_age" => 1800,
		"max_uses" => 1,
		"unique" => true,
		"temporary" => true
	);

	$result = $discord->RunAPI("POST", "channels/" . $channelid . "/invites", $options);
	if (!$result["success"])
	{
		var_dump($result);

		exit();
	}

	$url = "https://discord.gg/" . $result["data"]["code"];

	echo $url . "\n";
?>

Getting Started (Webhook)

Set up a webhook in Discord:

Go to Server Settings in a Discord server (must be the owner).
Integrations -> Webhooks -> New Webhook.
Name the webhook and assign it to a Discord channel.
Click the 'Copy Webhook URL' button.

Send a webhook notification that posts a message in a channel:

<?php
	require_once "support/sdk_discord.php";

	// Replace the string with the Webhook URL above.
	$url = "URL_OF_WEBHOOK";

	// For a complete list of options:
	// https://discord.com/developers/docs/resources/webhook#execute-webhook
	$options = array(
		"content" => "It works!"
	);

	$result = DiscordSDK::SendWebhookMessage($url, $options);
	if (!$result["success"])
	{
		var_dump($result);

		exit();
	}
?>

Send a webhook notification that posts a message with a file attachment in a channel:

<?php
	require_once "support/sdk_discord.php";

	// Replace the string with the Webhook URL above.
	$url = "URL_OF_WEBHOOK";

	// For a complete list of options:
	// https://discord.com/developers/docs/resources/webhook#execute-webhook
	$options = array(
		"content" => "It works!"
	);

	// Attaching a file.
	$fileinfo = array(
		"name" => "file",
		"filename" => "mycat.jpg",
		"type" => "image/jpeg",
		"data" => file_get_contents("/path/to/mycat.jpg")
	);

	// OR attach multiple files.
//	$fileinfo = array(
//		array(
//			"name" => "file",
//			"filename" => "mycat.jpg",
//			"type" => "image/jpeg",
//			"data" => file_get_contents("/path/to/mycat.jpg")
//		),
//		array(
//			"name" => "file2",
//			"filename" => "othercat.jpg",
//			"type" => "image/jpeg",
//			"data" => file_get_contents("/path/to/othercat.jpg")
//		),
//	);

	$result = DiscordSDK::SendWebhookMessage($url, $options, $fileinfo);
	if (!$result["success"])
	{
		var_dump($result);

		exit();
	}
?>

More Information

SDK documentation - The DiscordSDK class documentation.