The Concise Conditional "?"

Question

The first php script below does show "exactly seven", but the other one does not. Any idea why?

--- This one work ---

<?
$num = 7;
echo ($num == 7) ? "exactly seven" : "not seven";
?>

--- This one does not ---

<?
$num = 7;
echo ($num == 7) ? "exactly seven" : ($num > 7) ? "more than seven" : "less than seven";
?>

Answer 1

It's called precedence of operators. Add some parenthesis to ensure things happen in the right order, e.g.

$num = 7;
echo ($num == 7) ? "exactly seven" : (($num > 7) ? "more than seven" : "less than seven");

Answer 2

The result of the first part of the expression, ($num == 7) ? "exactly seven" : ($num > 7) is used as input for the other one.

So the expression can be read as echo "exactly seven" ? "more than seven" : "less than seven";.

Since "exactly seven" evaluates to boolean true, the echoed value is "more than seven".

Answer 3

As it's been said, it's in the precedence of operators. The behavior of your line of code

echo ($num == 7) ? "exactly seven" : ($num > 7) ? "more than seven" : "less than seven";

is called the Non-obvious ternary behavior. The evaluation happens from left to right so

1. echo (expr1) ? (expr2) : (expr3) ? (expr4) : (expr5)
2. evaluates the first ternary operator `?` to 
   echo (expr2) ? (expr4) : (expr5) 
   //because expr1 was TRUE ($num == 7)
3. and since the expr2 is "exactly seven" it finally evaluates to 
   echo (expr4).