• ?: is like a gate that lets anything falsy through – including NULL
• Anything falsy: 0, empty string, NULL, false, !isset(), empty()
• Same like old ternary operator: X ? Y : Z
• Note: ?: will throw PHP NOTICE on undefined (unset or !isset()) variables

Use of ?:

• checking empty(), !isset(), is_null() etc
• shorten ternary operation like !empty($x) ? $x : $y to echo $x ?: $y
• shorten if(!$x) { echo $x; } else { echo $y; } to echo $x ?: $y