?: operator (the 'Elvis operator') in PHP
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'
?>
By the way, it's called the Elvis operator.
See the docs:
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.
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 the future (in PHP 7) operator Null Coalesce Operator will do it, for example:
$param1 = $params['param1'] ?? null;
// Equivalent to: $param1 = isset($params['param1']) ? $params['param1'] : null;
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