I saw this today in some PHP code:
$items = $items ?: $this->_handle->result('next', $this->_result, $this); I'm not familiar with the ?: operator being used here. It looks like a ternary operator, but the expression to evaluate to if the predicate is true has been omitted. What does it mean?
It evaluates to the left operand if the left operand is truthy, and the right operand otherwise.
In pseudocode,
foo = bar ?: baz; roughly resolves to
foo = bar ? bar : baz; or
if (bar) { foo = bar; } else { foo = baz; } with the difference that bar will only be evaluated once.
You can also use this to do a "self-check" of foo as demonstrated in the code example you posted:
foo = foo ?: bar; This will assign bar to foo if foo is null or falsey, else it will leave foo unchanged.
Some more examples:
<?php var_dump(5 ?: 0); // 5 var_dump(false ?: 0); // 0 var_dump(null ?: 'foo'); // 'foo' var_dump(true ?: 123); // true var_dump('rock' ?: 'roll'); // 'rock' var_dump('' ?: 'roll'); // 'roll' var_dump('0' ?: 'roll'); // 'roll' var_dump('42' ?: 'roll'); // '42' ?> By the way, it's called the Elvis operator.
null or anything. Just sayin'
||. So blah || 'default'?
|| operator always returns a boolean.
var_dump('' ?: 'foo'); would be foo
__construct(MyClient $client = null) { $client = $client ?: new MyClient; }. When null is the only "falsey" option for variable, you're better of with $client ??= new MyClient;, which roughly translates to if ($client === null) { $client = new MyClient; }.See the docs:
Since PHP 5.3, it is possible to leave out the middle part of the ternary operator. Expression
expr1 ?: expr3returnsexpr1ifexpr1evaluates toTRUE, andexpr3otherwise.
expr2 is that it just disappeared, and isn't evaluated. $this->expensiveComputation() ?: "nope" is not identical to $this->expensiveComputation() ? $this->expensiveComputation() : "nope" - expr1 is only evaluated once.Be careful with arrays. We must write a checking variable after ?, because:
$params = ['param1' => 'value1', 'param2' => 'value2', 'param3' => 'value3',]; $param1 = isset($params['param1'])?:null; $param2 = !empty($params['param2'])?:null; $param3 = $params['param3']?:null; // get E_NOTICE, if $params['param3'] eq false var_dump($param1,$param2,$param3); true // would like to expect `value1` true // would like to expect `value2` param3 // properly, but problem above Updated
From RFC. In PHP 7 the operator Null Coalesce Operator will do it, for example:
$param1 = $params['param1'] ?? null; // Equivalent to: $param1 = isset($params['param1']) ? $params['param1'] : null; 
?: is the Elvis operator. This is a binary operator which does the following:
Coerces the value left of ?: to a boolean and checks if it is true. If true it will return the expression on the left side, if false it will return the expression on the right side.
var_dump(0 ?: "Expression not true"); // expression returns: Expression not true var_dump("" ?: "Expression not true"); // expression returns: Expression not true var_dump("hi" ?: "Expression not true"); // expression returns string hi var_dump(null ?: "Expression not true"); // expression returns: Expression not true var_dump(56 ?: "Expression not true"); // expression return int 56 The Elvis operator is basically shorthand syntax for a specific case of the ternary operator which is:
$testedVar ? $testedVar : $otherVar; The Elvis operator will make the syntax more consise in the following manner:
$testedVar ?: $otherVar; 
var_dump([] ?: 'Expression not true'); gives you "Expression not true". Therefore, the elvis operator is a bit like checking for empty() whereas the null coalesce operator (??) is more like testing isset().Another important consideration: The Elvis Operator breaks the Zend Opcache tokenization process. I found this the hard way! While this may have been fixed in later versions, I can confirm this problem exists in PHP 5.5.38 (with in-built Zend Opcache v7.0.6-dev).
If you find that some of your files 'refuse' to be cached in Zend Opcache, this may be one of the reasons... Hope this helps!
Yes, this is new in PHP 5.3. It returns either the value of the test expression if it is evaluated as TRUE, or the alternative value if it is evaluated as FALSE.
i think the purpose is conditional execution:
$a ?: func(); results in func() will only be executed if $a has a value that would resolve to FALSE. it may be used used as a short for
if(!$a){ func(); } thre assignment is optional $a = $a ?: func() is like:
if(!$a){ $a = func(); } 