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

user_242494 0
Viewed 190654

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?

language-construct conditional-operator php
7 Answers

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

user_1578820

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.

user_1181450

Elvis operator:

?: 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.

Example:

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

When to use:

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;
user_80594590

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.

user_2070820

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();
}
user_88892690
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