Understanding nested PHP ternary operator

You need to bracket the ternary conditionals:

<?php

for ($a=0; $a < 7; $a++) {
  echo (
    $a == 1 ? 'one' :
    ($a == 2 ? 'two' :
    ($a == 3 ? 'three' :
    ($a == 5 ? 'four' : 'other'))));
    echo "\n";
    // prints 'four'
}
exit;
?>

returns:

other
one
two
three
other
four
other

as you'd expect.

See the note at the bottom of "Ternary operators" at PHP Ternary operator help.

The expressions are being evaluated left to right. So you are actually getting:

  echo (
    ((($a == 1 ? 'one' : $a == 2)
     ? 'two' : $a == 3) ? 'three' :
    $a == 5) ? 'four' : 'other');

So for $a=2, you get:

  echo (
    ((($a==2) ? 'two' : $a == 3) ? 'three' :
    $a == 5) ? 'four' : 'other');

and then

  echo (
    ((true ? 'two' : $a == 3) ? 'three' :
    $a == 5) ? 'four' : 'other');

and then

  echo (
    ('two' ? 'three' : $a == 5) ? 'four' : 'other');

and then

  echo (
    'three' ? 'four' : 'other');

and so echo 'four'.

Remember that PHP is dynamically typed and treats any non-zero non-null values as TRUE.

On the Comparison Operators page in the PHP Manual they explain that PHP's behavior is "non-obvious" when nesting (stacking) ternary operators.

The code you've written is like this:

$a = 2;

echo
  ((($a == 1  ? 'one'   :
     $a == 2) ? 'two'   :
     $a == 3) ? 'three' :
     $a == 5) ? 'four'  : 
       'other'
  ;

// prints 'four'

As $a is 2 and both 'two' and 'three' are TRUE as well, you get "four" as the result, as you don't compare any longer if 'four' is TRUE or not.

If you want to change that, you have to put the brackets at different places [also noted by: BeingSimpler and MGwynne]:

$a = 2;
echo 
  ($a == 1 ? 'one'   :
  ($a == 2 ? 'two'   :
  ($a == 3 ? 'three' :
  ($a == 5 ? 'four'  : 
     'other'))))
  ;

// prints 'two'