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.

• 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 key issue here is operator precedence. In PHP, both the cast operator (int) and the comparison operator < have a higher precedence than the null coalescing operator ??. This means that in your first example

((int)$var['x']??0 < 1 )

PHP interprets the expression as if you had written:

(((int)$var['x']) ?? (0 < 1))

Here’s what happens step-by-step:

1. Casting: (int)$var['x'] is evaluated first, converting the string "5" to the integer 5.

2. Comparison: The < operator is evaluated next. Since < has a higher precedence than ??, the expression 0 < 1 is computed, yielding true.

3. Null Coalescing: Now PHP evaluates 5 ?? true. Because 5 is not null, the result of the null coalescing operator is 5.

4. Condition Check: In an if statement, a non-zero number is truthy, so the condition evaluates as true, and "less than 1" is printed.

In Example 2, a similar misinterpretation occurs:

($var['x']??5 < 1 )

This is interpreted as:

($var['x'] ?? (5 < 1))

Since $var['x'] is "5" (a non-null value), the null coalescing operator returns "5". In PHP, a non-empty string (other than "0") is truthy, so the condition is true, and "less" is printed.

In Example 3, you force the intended order by adding parentheses.

((int)($var['x']??0) < 1 )

Here, the expression inside the parentheses $var['x']??0 is evaluated first. Since $var['x'] exists, it returns "5", which is then cast to 5. The comparison 5 < 1 correctly evaluates to false, so "more than 1" is printed.

The unexpected behavior arises because without the proper grouping, PHP evaluates the cast and the comparison before applying the null coalescing operator. Adding explicit parentheses clarifies the order of operations and ensures the expression evaluates as intended.