• Elvis ?: returns the first argument if it contains a "true-ish" value (see which values are considered loosely equal to true in the first line of the Loose comparisons with == table). Or the second argument otherwise

    $result = $var ?: 'default';
    // is a shorthand for 
    $result = $var ? $var : 'default';
  

• Null coalescing ?? returns the first argument if it's set and is not null. Or the second argument otherwise

    $result = $var ?? 'default';
    // is a shorthand for 
    $result = isset($var) ? $var : 'default';
  

When your first argument is null, they're basically the same except that the null coalescing won't output an E_NOTICE when you have an undefined variable. The PHP 7.0 migration docs has this to say:

The null coalescing operator (??) has been added as syntactic sugar for the common case of needing to use a ternary in conjunction with isset(). It returns its first operand if it exists and is not NULL; otherwise it returns its second operand.

Here's some example code to demonstrate this:

<?php

$a = null;

print $a ?? 'b'; // b
print "\n";

print $a ?: 'b'; // b
print "\n";

print $c ?? 'a'; // a
print "\n";

print $c ?: 'a'; // Notice: Undefined variable: c in /in/apAIb on line 14
print "\n";

$b = array('a' => null);

print $b['a'] ?? 'd'; // d
print "\n";

print $b['a'] ?: 'd'; // d
print "\n";

print $b['c'] ?? 'e'; // e
print "\n";

print $b['c'] ?: 'e'; // Notice: Undefined index: c in /in/apAIb on line 33
print "\n";

The lines that have the notice are the ones where I'm using the shorthand ternary operator as opposed to the null coalescing operator. However, even with the notice, PHP will give the same response back.

Execute the code: https://3v4l.org/McavC

Of course, this is always assuming the first argument is null. Once it's no longer null, then you end up with differences in that the ?? operator would always return the first argument while the ?: shorthand would only if the first argument was truthy, and that relies on how PHP would type-cast things to a boolean.

So:

$a = false ?? 'f'; // false
$b = false ?: 'g'; // 'g'

would then have $a be equal to false and $b equal to 'g'.

The ??? operator has been dubbed the "empty coalesce" operator. It's similar to the "null coalesce" operator (??), but ignores empty strings ("") and empty arrays ([]) as well.

The empty coalescing operator isn't part of Craft (or Twig) core, which means your site likely has the Empty Coalesce or SEOmatic plugins installed; both of these plugins add this operator to Twig.

If you want to avoid using that non-standard operator, it can be replaced with the |default Twig filter:

{% set eventsButtonLabel = event.eventsButtonLabel|default('Book Ticket'|t) %}

(That last ??? null in your example seems redundant, unless there is a static translation for 'Book Ticket' that is explicitly set to an empty string, and you specifically need the eventsButtonLabel variable to be null if that is the case).