php "||" assignment logic compared to Javascript

Question

I was testing Heredoc and || functionality

function test()
{
    $obj = [(object)['col1'=>'val1'],
            (object)['col2'=>'val2'],
            (object)['col3'=>'val3']];
    $output = '';
$mystring = <<<EOT
a non empty string
EOT;

    foreach($obj as $prop)
    {
        foreach($prop as $i)
        {
            $output .= <<<EOT
            <div style='background-color:lightblue'> 
            \$head  : {gettype($i)} 
            </div>
EOT;
        }
    }
    $out =  $output || $mystring || 'couldn\'t find something valuable';
    echo $out;
}
test();

I get the output of

1

which represents a Boolean true. I had it output the contents of $mystring at one point by putting the logic in brackets eg

echo ($output || $mystring);

and it used to output:

a non empty string

It stopped working straight after and I am not sure what change broke it.

In javascript:

    function test()
    {
    
        var obj = [{'col1':'val1'},
                {'col2':'val2'},
                {'col3':'val3'}];
        var output = '';
        var mystring ="a non empty string";
    
        for(var prop in obj)
        {
            for(var i in prop)
            {
                output += 
                "<div style='background-color:lightblue'>" +
                    i + " : " + typeof prop[i] +
                    "</div>";
            }
        }
        out =  output || mystring || 'couldn\'t find something valuable';
        document.write(out);
        console.log("outputting \n\t" + out);
    }
    test();

It is slightly different from the php in terms of logic but the || during assignment works as expected. I get

0 : string

0 : string

0 : string

for the code above. If comment out the inner for loop like so

for(var prop in obj)
{
    /*for(var i in prop)
    {
        output += 
        "<div style='background-color:lightblue'>" +
        i + " : " + typeof prop[i] +
        "</div>";
    }*/
}

I get the contents of "mystring" which is

a non empty string

and if I then change mystring to an empty string like so

mystring = ""; //"a non empty string";

I get

couldn't find something valuable

How does php's "||" during assignment work ? Can someone explain it?

please fix your code formatting. function test() { and } test() is unformatted. — Gogol
– Gogol, Commented May 14, 2015 at 10:48
@noc2spamツ thanks. fixed it just before I read your comment.XD — Sarfaraaz
– Sarfaraaz, Commented May 14, 2015 at 13:02

Matteo Tassinari · Accepted Answer · 2015-05-14 10:58:15Z

2

If you assign a conditional expression in php you always get a boolean (that is, strictly true or false), it does not work as in javascript where the "truthy" value gets assigned, so in PHP

$a = "";
$b = 42;
$c = ($a || $b); // == true

While in javascript

var a = "";
var b = 42;
var c = (a || b); // == 42

and this is, sadly, by language design.

As @billyonecas reports, it is also in the comments to the docs: http://php.net/manual/en/language.operators.logical.php#77411

To obtain a similar behaviour in PHP, we must do something like:

$a = "";
$b = 42;
$c = ($a ?: $b); // newer php version
$c = ($a ? $a : $b); // older php version

if you need to concatenate expression, use parethesis:

$c = ($a ? $a : ($b ? $b : "not found"));

edited May 14, 2015 at 10:58

answered May 14, 2015 at 10:46

Matteo Tassinari

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6 Comments

billyonecan Over a year ago

php.net/manual/en/language.operators.logical.php#77411 and en.wikipedia.org/wiki/Short-circuit_evaluation

Sarfaraaz Over a year ago

Ok. but i did get it to work temporarily at one point by putting brackets around it. why could that be?

Matteo Tassinari Over a year ago

I really do not know, it just couldn't be as far as I know.

Sarfaraaz Over a year ago

I meant reproduce the result I got when using ||. but thanks. found that method with @KyleK in his comment section. it does allow the same logic though

KyleK Over a year ago

Only the inside parenthesis is needed in the nested ternary expression, assignation has not the precedence. See php.net/manual/en/language.operators.precedence.php

|

KyleK · Accepted Answer · 2015-05-14 11:15:00Z

1

With PHP, you can use ?:

$out =  $output ?: $mystring;

Or in older versions of PHP:

$out =  $output ? $output : $mystring;

You can also use empty() to get a more explicit code:

$out =  empty($output) ? $mystring : $output;

http://php.net/manual/en/function.empty.php

Note: these ternary expressions are equivalent to if-else:

if(empty($output))
{
    $out = $mystring;
}
else
{
    $out = $output;
}

With nested ternary, add parentheses, as the default precedence is:

$out =  $output ? $output : $mystring ? $mystring : 'default';

This is:

$out =  ($output ? $output : $mystring) ? $mystring : 'default';

So you must do:

$out =  $output ? $output : ($mystring ? $mystring : 'default');

edited May 14, 2015 at 11:15

answered May 14, 2015 at 10:35

KyleK

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5 Comments

Sarfaraaz Over a year ago

you say empty() gets more explicit code. like the kind of output you would get form var_dump() or json_encode()? <-(more descriptive easier to read)

Sarfaraaz Over a year ago

I tried $out = $output? $output : $mystring ? $mystring : 'couldnt find something valuable'; just before I read your post. I like the newer php version you have enlightened me to. I got the content of $mystring though(a non empty string) instead of the content of $output. not sure why

KyleK Over a year ago

Just add the parentheses: $out = $output ? $output : ($mystring ? $mystring : 'couldnt find something valuable'); and I do not understand your first comment.

Sarfaraaz Over a year ago

about my first comment : sorry was just asking if what the output of empty() looks like but I will look it up

KyleK Over a year ago

Done, for functions please see the doc, empty return true or false, it why it is more explicit to use it in a condition.

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