New sniff: implement null coalescing operator in select ternaries

[jrfnl]

Sniff basics	-
Fixable for PHP:	7.0+
Sniff type:	Modernize
Fixer type:	Safe/Risky

Short description

PHP 7.0 introduced the null coalescing operator to allow for abbreviating typical isset($x) ? $x : $y ternaries.

Related PHPCompatibility sniff(s):

• NewLanguageConstructs

PHP manual references:

• 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.

  http://php.net/manual/en/migration70.new-features.php#migration70.new-features.null-coalesce-op

• http://php.net/manual/en/language.operators.comparison.php#language.operators.comparison.coalesce

Example code:

Detect the following code pattern(s):

$action = isset($_POST['action']) ? $_POST['action'] : 'default';

if (isset($_POST['action'])) {
    $action = $_POST['action'];
} else {
    $action = 'default';
}

$username = isset($_GET['user']) ? $_GET['user'] : 'nobody';

And fix these to:

$action = $_POST['action'] ?? 'default';

// The above is identical to this if/else statement
$action = $_POST['action'] ?? 'default';

$username = $_GET['user'] ?? 'nobody';

Notes for implementation of the sniff:

• Whether or not to shorten if/else statements which comply with this pattern should probably be decided based on a public property which can be set from within a custom ruleset.
• Beware of nested ternaries.

Prior art:

• CS-Fixer: ternary_to_null_coalescing