Difference between "?:" and "??" operator in PHP

?? operator

•  Never cause  to undefined warning error
• If on left side value is null or undefined returns right side of operator, in other cases returns value on left side. 

• echo null ?? "right" ;   // right
• echo $adsaada ?? "right"; //right
• echo "" ?? "right"; // ""
• echo 0 ?? "right"; // 0
• echo true ?? false; // 1

"?:" operator -actually this is a bit more shorthand of ternary operator. Checks whether result is true or not. Also returns undefined warning error if not exists  

• echo null ?? "right"; // right
• echo $not_exists_var ??  // undefined warning error then  "right";
• echo "" ?? "right"; // ""
• echo 0 ?? "right"; // "right"
• echo true ?? false; 1