PHP ternary operator vs null coalescing operator

Can someone explain the differences between ternary operator shorthand (?:) and null coalescing operator (??) in PHP?

When do they behave differently and when in the same way (if that even happens)?

$a ?: $b

VS.

$a ?? $b

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'.

Ran the below on php interactive mode (php -a on terminal). The comment on each line shows the result.

var_export (false ?? 'value2');   // false
var_export (true  ?? 'value2');   // true
var_export (null  ?? 'value2');   // value2
var_export (''    ?? 'value2');   // ""
var_export (0     ?? 'value2');   // 0

var_export (false ?: 'value2');   // value2
var_export (true  ?: 'value2');   // true
var_export (null  ?: 'value2');   // value2
var_export (''    ?: 'value2');   // value2
var_export (0     ?: 'value2');   // value2

The Null Coalescing Operator ??

• ?? is like a "gate" that only lets NULL through.
• So, it always returns first parameter, unless first parameter happens to be NULL.
• This means ?? is same as ( !isset() || is_null() )

Use of ??

• shorten !isset() || is_null() check
• e.g $object = $object ?? new objClassName();

Stacking Null Coalese Operator

        $v = $x ?? $y ?? $z; 

        // This is a sequence of "SET && NOT NULL"s:

        if( $x  &&  !is_null($x) ){ 
            return $x; 
        } else if( $y && !is_null($y) ){ 
            return $y; 
        } else { 
            return $z; 
        }

The Ternary Operator ?:

• ?: is like a gate that lets anything falsy through - including NULL
• Anything falsy: 0, empty string, NULL, false, !isset(), empty()
• Same like old ternary operator: X ? Y : Z
• Note: ?: will throw PHP NOTICE on undefined (unset or !isset()) variables

Use of ?:

• checking empty(), !isset(), is_null() etc
• shorten ternary operation like !empty($x) ? $x : $y to $x ?: $y
• shorten if(!$x) { echo $x; } else { echo $y; } to echo $x ?: $y

Stacking Ternary Operator

        echo 0 ?: 1 ?: 2 ?: 3; //1
        echo 1 ?: 0 ?: 3 ?: 2; //1
        echo 2 ?: 1 ?: 0 ?: 3; //2
        echo 3 ?: 2 ?: 1 ?: 0; //3
    
        echo 0 ?: 1 ?: 2 ?: 3; //1
        echo 0 ?: 0 ?: 2 ?: 3; //2
        echo 0 ?: 0 ?: 0 ?: 3; //3

    
        // Source & Credit: http://php.net/manual/en/language.operators.comparison.php#95997
   
        // This is basically a sequence of:

 
        if( truthy ) {}
        else if(truthy ) {}
        else if(truthy ) {}
        ..
        else {}

Stacking both, we can shorten this:

        if( isset($_GET['name']) && !is_null($_GET['name'])) {
            $name = $_GET['name'];
        } else if( !empty($user_name) ) {
             $name = $user_name; 
        } else {
            $name = 'anonymous';
        }

To this:

        $name = $_GET['name'] ?? $user_name ?: 'anonymous';

Cool, right? :-)

If you use the shortcut ternary operator like this, it will cause a notice if $_GET['username'] is not set:

$val = $_GET['username'] ?: 'default';

So instead you have to do something like this:

$val = isset($_GET['username']) ? $_GET['username'] : 'default';

The null coalescing operator is equivalent to the above statement, and will return 'default' if $_GET['username'] is not set or is null:

$val = $_GET['username'] ?? 'default';

Note that it does not check truthiness. It checks only if it is set and not null.

You can also do this, and the first defined (set and not null) value will be returned:

$val = $input1 ?? $input2 ?? $input3 ?? 'default';

Now that is a proper coalescing operator.