C#'s null coalescing operator (??) in PHP

Question 1

Is there a ternary operator or the like in PHP that acts like ?? of C#?

?? in C# is clean and shorter, but in PHP you have to do something like:

// This is absolutely okay except that $_REQUEST['test'] is kind of redundant.
echo isset($_REQUEST['test'])? $_REQUEST['test'] : 'hi';

// This is perfect! Shorter and cleaner, but only in this situation.
echo null? : 'replacement if empty';

// This line gives error when $_REQUEST['test'] is NOT set.
echo $_REQUEST['test']?: 'hi';

Question 2

PHP 7 adds the null coalescing operator:

// Fetches the value of $_GET['user'] and returns 'nobody'
// if it does not exist.
$username = $_GET['user'] ?? 'nobody';
// This is equivalent to:
$username = isset($_GET['user']) ? $_GET['user'] : 'nobody';

You could also look at short way of writing PHP's ternary operator ?: (PHP >=5.3 only)

// Example usage for: Short Ternary Operator
$action = $_POST['action'] ?: 'default';

// The above is identical to
$action = $_POST['action'] ? $_POST['action'] : 'default';

And your comparison to C# is not fair. "in PHP you have to do something like" - In C# you will also have a runtime error if you try to access a non-existent array/dictionary item.

Question 3

Update

JavaScript now supports the nullish coalescing operator (??). It returns its right-hand-side operand when its left-hand-side operand is null or undefined, and otherwise returns its left-hand-side operand.

Please check compatibility before using it.

The JavaScript equivalent of the C# null coalescing operator (??) is using a logical OR (||):

var whatIWant = someString || "Cookies!";

There are cases (clarified below) that the behaviour won't match that of C#, but this is the general, terse way of assigning default/alternative values in JavaScript.

Clarification

Regardless of the type of the first operand, if casting it to a Boolean results in false, the assignment will use the second operand. Beware of all the cases below:

alert(Boolean(null)); // false
alert(Boolean(undefined)); // false
alert(Boolean(0)); // false
alert(Boolean("")); // false
alert(Boolean("false")); // true -- gotcha! :)

This means:

var whatIWant = null || new ShinyObject(); // is a new shiny object
var whatIWant = undefined || "well defined"; // is "well defined"
var whatIWant = 0 || 42; // is 42
var whatIWant = "" || "a million bucks"; // is "a million bucks"
var whatIWant = "false" || "no way"; // is "false"

Question 4

Sadly - no. The closest you can do is:

int y = (x != null) ? x : -1;

Of course, you can wrap this up in library methods if you feel the need to (it's unlikely to cut down on length much), but at the syntax level there isn't anything more succinct available.

Question 5

This happens as consequence of the assignment operator also returning the value:

The assignment operator (=) stores the value of its right-hand operand in the storage location, property, or indexer denoted by its left-hand operand and returns the value as its result.

The expression b = 12 not only assigns 12 to b, but also returns this value.

Question 6

Thanks to @Devon and @Dormilich for their responses.

TL;DR: You can't use the null coalescing operator (??) with constants. You have to use defined() instead.

According to the documentation for the null coalescing operator (??):

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.

Meaning that $x ?? $y is shorthand for isset($x) ? $x : $y. And this is where the problem lies, because the documentation for isset explicitly states:

Warning: isset() only works with variables as passing anything else will result in a parse error. For checking if constants are set use the defined() function.

That's what throws the fatal php error I describe in the question. Instead, a solution would be to do away with the null coalescing operator and replace it with defined():

class a {
    private const SOMETHING = 'This is a!';

    public static function outputSomething() {
        return defined('static::SOMETHING') ? static::SOMETHING : self::SOMETHING;
    }
}

class b extends a {
    //protected const SOMETHING = 'This is b!';
}

echo (new b())::outputSomething();

A second solution is to change how the code works in the first place. As @Devon correctly points out, the private visibility of a::SOMETHING prevents class b from seeing it, so b::SOMETHING is not defined. However, when the visibility of a::SOMETHING is changed to protected, class b can see it and b::SOMETHING references it. This code doesn't need the null coalescing operator at all, and can just use static::SOMETHING without any conditional:

class a {
    protected const SOMETHING = 'This is a!';

    public static function outputSomething() {
        return static::SOMETHING;
    }
}

class b extends a {
    //protected const SOMETHING = 'This is b!';
}

echo (new b())::outputSomething();

C#'s null coalescing operator (??) in PHP

Answers

Clarification

php,ternary-operator,null-coalescing-operator,php-7