From the docs:

Coalesce equal or ??=operator is an assignment operator. If the left parameter is null, assigns the value of the right parameter to the left one. If the value is not null, nothing is done.

Example:

// The following lines are doing the same
$this->request->data['comments']['user_id'] = $this->request->data['comments']['user_id'] ?? 'value';
// Instead of repeating variables with long names, the equal coalesce operator is used
$this->request->data['comments']['user_id'] ??= 'value';

So it's basically just a shorthand to assign a value if it hasn't been assigned before.

No, there's no non-falsy null coalesce operator for PHP, but there's a workaround. Meet ??0?: :

<?php

$truly = true; // anything truly
$falsy = false; // anything falsy (false, null, 0, '0', '', empty array...)
$nully = null;

// PHP 7's "null coalesce operator":
$result = $truly ?? 'default'; // value of $truly
$result = $falsy ?? 'default'; // value of $falsy
$result = $nully ?? 'default'; // 'default'
$result = $undef ?? 'default'; // 'default'

// but because that is so 2015's...:
$result = !empty(foo : 'default';

// ... here comes...
// ... the "not falsy coalesce" operator!
$result = $truly ??0?: 'default'; // value of $truly
$result = $falsy ??0?: 'default'; // 'default'
$result = $nully ??0?: 'default'; // 'default'
$result = $undef ??0?: 'default'; // 'default'

// explanation:
($foo ?? <somethingfalsy>) ?: 'default';
($foo if set, else <somethingfalsy>) ? ($foo if truly) : ($foo if falsy, or <somethingfalsy>);

// here is a more readable[1][2] variant:
??''?:

// [1] maybe
// [2] also, note there is a +20% storage requirement

Source:
https://gist.github.com/vlakoff/890449b0b2bbe4a1f431

But do your team & yourself a favor and just “don’t”.

• 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'.

Null coalescing operator returns its first operand if it exists and is not NULL;

Otherwise it returns its second operand.

In your case first operand is false so it is assigned to the variable. For e.g; if you initialize null to first operand then it will assign second operands value as shown.

 $a = null;
 $x = $a ?? 'abc';
 var_dump($x);

Result :
string(3) "abc"