Expressions And Operators: Conditional

The ternary operator ?: takes three operands. Based on the value of the first operand, either the second or third operand—but not both—is evaluated, and their result becomes the result of the whole expression. For example:

$daysInFeb = is_leap_year($year) ? 29 : 28

Here, $daysInFeb takes on the value 29 or 28, depending on the truth of the result from the call to function is_leap_year. Here's another example:

for ($i = -5; $i <= 5; ++$i) {
  echo "$i is ". ((($i & 1) === true) ? "odd" : "even") . "\n";

If the low-order bit of an integer is set, the value is odd; otherwise, it's even. So, the first operand, ($i & 1 === true), is evaluated. If it is true, "odd" becomes the result; otherwise, "even" becomes the result.

There is a sequence point after the evaluation of the first operand, so any side-effects in that expression are completed before the second or third operand is evaluated. For example:

$i++ ? f($i) : g($i * 2); // the sequence point makes this well-defined

If the second operand is omitted, the result and type of the whole expression is the value and type of the first expression when it was tested. For example:

$a = 10 ?: "Hello";  // result is int with value 10
$a = 0 ?: "Hello";   // result is string with value "Hello"

Consider the following:

process($a > $b ? 10 : 20, $x !== $y ? f($x) : g($y));

Writing this function call without using the conditional operator requires a series of nested if statements, and if there are more conditionally determined arguments, it would be even more unwieldy! That said, sometimes it might be more overt to use the nested-if approach. Compare the following, alternative approaches:

function factorial1(int $int): int
  return ($int > 1) ? $int * factorial1($int - 1) : $int;

function factorial2(int $i): int {
  if ($i > 1) return $i * factorial2($i - 1);
  else if ($i === 1) return $i;
  else return 0;