php 7 - PHP ternary operator vs null coalescing operator -

can explain differences between ternary operator shorthand (?:) , null coalescing operator (??) in php?

when behave differently , when in same way (if happens)?

$a ?: $b 

vs.

$a ?? $b 

they're same except null coalescing won't output e_notice when have undefined variable.

here's test code:

<?php  $a = null;  print $a ?? 'b'; print "\n";  print $a ?: 'b'; print "\n";  print $c ?? 'a'; print "\n";  print $c ?: 'a'; print "\n";  $b = array('a' => null);  print $b['a'] ?? 'd'; print "\n";  print $b['a'] ?: 'd'; print "\n";  print $b['c'] ?? 'e'; print "\n";  print $b['c'] ?: 'e'; print "\n"; 

and it's output:

b b  notice: undefined variable: c in /in/apaib on line 14 d d e  notice: undefined index: c in /in/apaib on line 33 e 

the lines have notice ones i'm using shorthand terninary operator opposed null coalescing. however, notice, php give same response back.

execute code: https://3v4l.org/mcavc

of course, assuming first argument null. once it's no longer null, end differences in ?? operator return first argument while ?: shorthand if first argument truthy, , relies on how php type-cast things boolean.

so:

$a = false ?? 'f'; $b = false ?: 'g'; 

would have $a equal false , $b equal 'g'.