PHP Ternary Operators don't work as expected?

Question

I have a method that I'm checking if param is null, but if I use a ternary operator to make sure the false result isn't a string, I don't get the same expected result... I am a full stack .NET dev by day, but do some PHP free lance and this just stumped me...

$param = null;

// $active evaluates to true
$active = is_null($param) ? true : false;

// $active evaluates to false
$active = is_null($param) ? true : is_string($param)
    ? (strtolower($param) === 'true')
    : true;

I have used nested ternary operators in C# and JavaScript what feels like countless times, but I don't know if I have ever tried in PHP... does PHP attempt to evaluate all nested ternary operations prior to expressing a result or is there something I'm missing here since from my understanding in this case, the ternary operator should be short circuited and evaluated to true in both circumstances.

Answer 1

You need to wrap your second ternary condition with parenthesis (),

<?php
$param = null;
// $active evaluates to true
$active = is_null($param) ? true : false;
echo "Simple ternary result = $active".PHP_EOL;
// $active evaluates to true
$active = is_null($param) ? true : (is_string($param)? (strtolower($param) === 'true'): true);
echo "Nested ternary result = $active";
?>

Note:

It is recommended that you avoid "stacking" ternary expressions. PHP's behaviour when using more than one ternary operator within a single statement is non-obvious:

See Example #4 here at http://php.net/manual/en/language.operators.comparison.php

Example #4 Non-obvious Ternary Behaviour

<?php
// on first glance, the following appears to output 'true'
echo (true?'true':false?'t':'f');

// however, the actual output of the above is 't'
// this is because ternary expressions are evaluated from left to right

// the following is a more obvious version of the same code as above
echo ((true ? 'true' : false) ? 't' : 'f');

// here, you can see that the first expression is evaluated to 'true', which
// in turn evaluates to (bool)true, thus returning the true branch of the
// second ternary expression.
?>

DEMO: https://3v4l.org/gW8pk

Answer 2

The ternary operator is left associative unlike most other languages such as C#. The code:

$active = is_null($param)
    ? true
    : is_string($param)
        ? (strtolower($param) === 'true')
        : true;

is evaluated as follows:

$active = ((is_null($param) ? true : is_string($param))
                                                        ? (strtolower($param) === 'true') : true);

You must explicitly add parenthesis to make sure ?: works the way it does in familiar languages:

$active = is_null($param)
    ? true
    : (is_string($param)
        ? (strtolower($param) === 'true')
        : true);

Answer 3

This is a well-known problem with PHP. I doubt it will ever be fixed. Use parentheses, or if..else or switch statements, to get the behaviour you want.

(In technical terms, the ternary operator in PHP is "left associative", while that in every other language with this operator is "right associative". The latter is the more logical behaviour for this operator.)