how to use ?: in php code example

Example 1: php != operator

$a == $b	Equal	TRUE if $a is equal to $b after type juggling.
$a === $b	Identical	TRUE if $a is equal to $b, and they are of the same type.
$a != $b	Not equal	TRUE if $a is not equal to $b after type juggling.
$a <> $b	Not equal	TRUE if $a is not equal to $b after type juggling.
$a !== $b	Not identical	TRUE if $a is not equal to $b, or they are not of the same type.
$a < $b	Less than	TRUE if $a is strictly less than $b.
$a > $b	Greater than	TRUE if $a is strictly greater than $b.
$a <= $b	Less than or equal to	TRUE if $a is less than or equal to $b.
$a >= $b	Greater than or equal to	TRUE if $a is greater than or equal to $b.
$a <=> $b	Spaceship	An integer less than, equal to, or greater than zero when $a is less than, equal to, or greater than $b, respectively. Available as of PHP 7.

Example 2: php ?? vs ?:

<?php
var_dump(null ?? '`null` is null'); // `null` is null
var_dump(null ?: '`null` is empty'); // null
var_dump(true ?? '`true` is null'); // true
var_dump(true ?: '`true` is empty'); // true
var_dump(false ?? '`false` is null'); // false
var_dump(false ?: '`false` is empty'); // `false` is empty"
var_dump('' ?? 'empty string is null'); // ""
var_dump('' ?: 'empty string is empty'); // "empty string is empty"
var_dump(empty(null) ?: '`null` is not empty'); // true