?: operator (the 'Elvis operator') in PHP – Dev

The best answers to the question “?: operator (the 'Elvis operator') in PHP” in the category Dev.

QUESTION:

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?

ANSWER:

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.

ANSWER:

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.

Elvis operator

ANSWER:

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!

ANSWER:

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;