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The ternary operator can be thought of as an inline if statement. It consists of three parts. The operator, and two outcomes. The syntax is as follows:

Example:

$value = <operator> ? <true value> : <false value>

If the operator is evaluated as true, the value in the first block will be returned (<true value>), else the value in the second block will be returned (<false value>). Since we are setting $value to the result of our ternary operator it will store the returned value.

Example:

$action = empty($_POST['action']) ? 'default' : $_POST['action'];

$action would contain the string 'default' if empty($_POST['action']) evaluates to true. Otherwise it would contain the value of $_POST['action'].

The expression (expr1) ? (expr2) : (expr3) evaluates to expr2 if expr1evaluates to true, and expr3 if expr1 evaluates to false.

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. ?: is often referred to as Elvis operator.

This behaves like the Null Coalescing operator ??, except that ?? requires the left operand to be exactly null while ?: tries to resolve the left operand into a boolean and check if it resolves to boolean false.

Example:

function setWidth(int $width = 0){
 $_SESSION["width"] = $width ?: getDefaultWidth();
}

In this example, setWidth accepts a width parameter, or default 0, to change the width session value. If $width is 0 (if $width is not provided), which will resolve to boolean false, the value of getDefaultWidth() is used instead. The getDefaultWidth() function will not be called if $width did not resolve to boolean false.

Refer to Types for more information about conversion of variables to boolean.