New sniff: implement short ternary in select ternaries

[jrfnl]

Sniff basics	-
Fixable for PHP:	5.3+
Sniff type:	Modernize
Fixer type:	Risky

Short description

PHP 5.3 introduced shorthand ternaries where the middle part can be left out.

Related PHPCompatibility sniff(s):

• TernaryOperators

PHP manual references:

• Since PHP 5.3, it is possible to leave out the middle part of the ternary operator. Expression expr1 ?: expr3 returns expr1 if expr1 evaluates to TRUE, and expr3 otherwise.

  http://php.net/manual/en/language.operators.comparison.php

Example code:

Detect the following code pattern(s):

$a = $start ? $start : 0;

$can_drink = ( $age >= 21 ) 
  ? true 
  : false ;

$job_title = array_search( "Some Name", $emp )
  ? array_search( "Some Name", $emp )
  : "n/a" ;

// Don't fix:
$a = ( $start > 0 ) ? $start : 0;

And fix these to:

$a = $start ?: 0;

$can_drink = ( $age >= 21 ) 
  ?: false ;

$job_title = array_search( "Some Name", $emp )
  ?: "n/a" ;

Notes for implementation of the sniff:

• Basically the sniff would need to verify that the part between the ? and the : is the same as the condition or true where the condition can only evaluate to a boolean.
• Beware of nested ternaries.