Conditional Operator Associativities

In Ruby:

a = true ? :a : true ? :b : :c
p a

Guess the answer! Yes, as you thought, the answer is :a.

In JavaScript:

var a = true ? 'a' : true ? 'b' : 'c';
alert(a);

Guess the answer! Yes, as you thought, the answer is a.

In Vim script:

let a = 1 ? 'a' : 1 ? 'b' : 'c'
echo a

Guess the answer! Yes, as you thought, the answer is a.

In PHP:

$a = true ? 'a' : true ? 'b' : 'c';
print_r($a);

Guess the answer! No, the answer if "b".

$a = true ? 'a' : (true ? 'b' : 'c');
print_r($a);

The code above is the equivalent code to the examples in Ruby, JS and Vim script. It's impossibly difficult to imagine why the specification is so.