What's the Difference Between ?? and ?: in PHP?

?: (Elvis Operator)

The elvis operator (?:) is actually a name used for shorthand ternary (which was introduced in PHP 5.3). It has the following syntax:

// PHP 5.3+
expr1 ?: expr2;

This is equivalent to:

expr1 ? expr1 : expr2;

?? (Null Coalescing Operator)

The null coalescing operator (??) was introduced in PHP 7, and it has the following syntax:

// PHP 7+
$x ?? $y;

This is equivalent to:

isset($x) ? $x : $y;

?: vs. ??

The table below shows a side-by-side comparison of the two operators against a given expression:

Expression	echo ($x ?: 'hello')	echo ($x ?? 'hello')
$x = "";	'hello'	""
$x = null;	'hello'	'hello'
$x;	'hello' (and Notice: Undefined variable: x)	'hello'
$x = [];	'hello'	[]
$x = ['a', 'b'];	['a', 'b']	['a', 'b']
$x = false;	'hello'	false
$x = true;	true	true
$x = 1;	1	1
$x = 0;	'hello'	0
$x = -1;	-1	-1
$x = '1';	'1'	'1'
$x = '0';	'hello'	'0'
$x = '-1';	'-1'	'-1'
$x = 'random';	'random'	'random'
$x = new stdClass;	object(stdClass)	object(stdClass)