Twilio Developer Evangelist here. Since this post has 13k+ views and no official answer (hidden in
comments), I’d like to resolve this for those that have the same
issue.

As @dwhitz mentions, HTTP 401 is an Unauthorized Error (aka no authorization).

If you run into the same error, that likely means your credentials aren’t set up correctly.

Step 1: Locate Credentials

All requests to Twilio’s REST API need to be authenticated. Twilio supports two forms of authentication (both using HTTP basic auth), so you can pick either option:

1. Account SID & Account Token

These can be found on your Account Dashboard on the Twilio Console.

2. API Keys

You can create an API Key within Settings->API Keys on your Twilio console.

Step 2: Secure Credentials

You should now have either an (Account SID + Account Token) or an (API Key SID + API Key Secret). These pieces of information should be hidden and not publicly available (don’t share them with your co-workers or push them to GitHub).

Secure them by storing them in a way that prevents unauthorized access, like environment variables. Here are examples using the 1st form (Account SID & Account Token).

Via Mac/Linux

echo "export TWILIO_ACCOUNT_SID='ACXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'" > twilio.env
echo "export TWILIO_AUTH_TOKEN='your_auth_token'" >> twilio.env
source ./twilio.env

Make sure to add twilio.env to your .gitignore

echo "twilio.env" >> .gitignore

Via Windows

set TWILIO_ACCOUNT_SID=ACXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
set TWILIO_AUTH_TOKEN=your_auth_token

Step 3: Load Your Credentials

If you’re getting the HTTP 401 error, this is likely the biggest point of failure. You’ll want to make sure you’re loading the credentials properly.

Now, this depends on your language, but since this question is in PHP I’ll answer it in that language using getenv().

If you’re using another language, check out how you can do it in
Node.js,
C#,
Ruby,
Python,
or
Java.

$sid = getenv('TWILIO_ACCOUNT_SID');
$token = getenv('TWILIO_AUTH_TOKEN');

After this, you’ll initialize your Twilio Client like so:

$client = new Client($sid, $token);

PS. There are other ways to work with environment variables in PHP, so check out this blog post if you’d like to do it another way.

Step 4: Still Not Working?

At this point, if you’re still getting HTTP 401 there’s likely something wrong with how you’re storing/reading the credentials via environment variables.

To debug, you could add print/echo statements to output the data of your env variables. If no data is present, you might want to do more research on working with environment variables.