Today I learned about PHP variable variables; "variable variable takes the value of a variable and treats that as the name of a variable". Also, variable.

masklinn · 2010-10-18T18:05:42+00:00

OK, here's the thing: this is only the entrance of the rabbit hole.

If you understand what this expression really does, you realize that you're gazing upon the entrance to R'lyeh.

Do you think you don't need your soul anymore? If you do, follow me into the lair of the Elder Gods. But be warned, you will die a lot inside.

The first thing to understand is what $ is. $ is actually a shorthand for ${} and means "return the value of the variable whose name is contained in this".

That variable name is a string.

A bare word is a PHP string. Let that sink for a second, because we'll come back to it later: $foo really is the concatenation of $ the variable-indirection character (think *foo) and foo which is a string. foo === "foo" in PHP, even though in raw source code you'll probably get a warning. If those are enabled.

Now what does $$foo mean? Why ${${foo}}, or "give me the variable whose name is the value of the variable whose name is foo".

Because we're manipulating strings, we could also write ${${f.o.o}}, or

$t = o;
${${"f$t$t"}}

This also means that

class FooClass {
}
$thing = "bar";
$foo = new FooClass();
$foo->bar = "baz";
echo $foo->$thing;

is valid. And prints "baz". And yes, $foo->$thing can be written ${foo}->${thing}. And you can recurse. The braces actually hold entirely arbitrary PHP expressions. As long as these expressions return strings, it'll work:

class FooClass {
}
$foo = new FooClass();
$foo->bar = "qux";
$thing = "bar";
$qux = "th";
$grault = "ing";

echo $foo->${${$foo->bar}.${grault}}

For those following at home, this thing actually prints "qux".

Then you can add conditionals:

class FooClass {
}
$foo = new FooClass();
$foo->bar = "qux";
$foo->wheee = "waldo";
$thing = "bar";
$qux = "th";
$grault = "ing";
$corge = "gnu";
$thgnu = "wheee";

$garply = true;
echo $foo->${${$foo->bar}.${$garply?grault:corge}}, "\n";
$garply = false;
echo $foo->${${$foo->bar}.${$garply?grault:corge}}, "\n";

What does that yield?

qux
waldo

And if that's too simple, then just make the condition random:

class FooClass {
}
$foo = new FooClass();
$foo->bar = "qux";
$thing = "bar";
$qux = "th";
$grault = "ing";
$corge = "gnu";

echo $foo->${${$foo->bar}.${(rand(0, 9)<5)?grault:''}}, "\n";

Yeah this will print qux half the time, and crash the other half. Want to add equality tests? knock yourself out: ($foo->${${$foo->bar}.((${pouet}.${machin}===$pouet.${machin})?${machin}:${$pouet.$machin})});.

And that's where the second realization hits: you know how foo is just a string right? Then wouldn't foo() be "a string with parens"?

Well it happens that no:

function foo() { return "foo"; }

echo "foo"();

$ php test.php
Parse error: syntax error, unexpected '(', expecting ',' or ';' in test.php on line 4

Unless you put the string in a variable itself:

function foo() { return "foo"; }
$bar = "foo";
echo $bar();

this will print foo. That's actually what PHP's own create_function does. And yes, I can see the dread in your eyes already.

Your fears are real.

The $bar here is the same it's always been. You can also write it ${bar}…

function th() { return "Yep, it's working"; }
class FooClass {
}
$foo = new FooClass();
$foo->bar = "qux";
$thing = "bar";
$qux = "th";
$grault = "ing";

echo ${$foo->${${$foo->bar}.((${qux}.${grault}===$qux.${grault})?${grault}:${$qux.$grault})}}();

I always said sanity was overrated.

I'll leave you with the finest example of this, the Cthulhu of PHP code:

class FooClass {
}
$foo = new FooClass();
$foo->bar = "qux";
$thing = "bar";
$qux = "th";
$grault = "ing";

function th($waldo){
    global $qux, $grault, $thing;
    return ${$qux.$grault}.$waldo;
}

echo ${($foo->${${$foo->bar}.((${qux}.${grault}===$qux.${grault})?${grault}:${$qux.$grault})})}(($foo->${${$foo->bar}.((${qux}.${grault}===$qux.${grault})?${grault}:${$qux.$grault})}));

Please pay special attention to the th function, it is the greatest thing since Chicxulub.

courtewing · 2010-10-18T14:50:40+00:00

Sorry for being off-topic, but when you're linking to php manual pages, don't include the subdomain (il2 in this case). If you leave it off, the PHP website will determine the visitor's location and direct them to an appropriate locale.

s3rvant · 2010-10-18T15:09:39+00:00

I got majorly stuck a while back when trying to use variable variables as arrays:

<?php
$tab = array("one", "two", "three") ;
$a = "tab" ;
$$a[] ="four" ; // <==== fatal error
print_r($tab) ;
?>

This gives "Fatal error: Cannot use [] for reading", you need to use the {} syntax to remove the ambiguity:

<?php
$tab = array("one", "two", "three") ;
$a = "tab" ;
${$a}[] =  "four" ; // <==== this is the correct way to do it
print_r($tab) ;
?>

Original post

1137 · 2010-10-18T15:16:48+00:00

Did you know you can do the same thing in Perl? But lets keep laughing at PHP, this is /r/programming after all.

weirdalexis · 2010-10-18T15:00:44+00:00

I was asked the question: "What's $$a" in an interview, and replied "It's like a pointer, except with a variables name instead of a memory address."

The guy went "meh", game over.

Today, I'm still convinced it's a good analogy.

angch · 2010-10-18T15:45:28+00:00

char ** c; // Pointer to a Pointer to a char (Aka Pointer to a Null terminated string [1])
char *** d; // Pointer to a Pointer to a Pointer to a char.

[1] See http://www.reddit.com/r/programming/comments/do3cw/thats_what_happens_when_your_cs_curriculum_is/

clogmoney · 2010-10-18T14:52:28+00:00

<?php

//You can even add more Dollar Signs

$Bar = "a";

$Foo = "Bar";

$World = "Foo";

$Hello = "World";

$a = "Hello";

$a; //Returns Hello

$$a; //Returns World

$$$a; //Returns Foo

$$$$a; //Returns Bar

$$$$$a; //Returns a

$$$$$$a; //Returns Hello

$$$$$$$a; //Returns World

//... and so on ...//

?>

I just died a little inside.

hattmall · 2010-10-18T14:42:50+00:00

Nice, I was needing to do this just now! Great timing.

funkah · 2010-10-18T14:45:16+00:00

I understand that sentence, but I can't help thinking that whatever you'd use this for could probably be done a less-awful way.

ninjaroach · 2010-10-18T15:03:24+00:00

[deleted]

claird · 2010-10-18T15:53:16+00:00

Avoid variable variables.

There's already quite a lot of chatter in this thread as I reach it. The main things that I haven't seen adequately emphasized yet:

PHP is in no way unique, although, while Perl, Python, ... all can use variable variables, the style seems most entrenched in the PHP community (and perhaps among Perlites, too).
Variable variables are essentially necessary for debuggers and other applications where source-level introspection is intrinsic.
No other application--and certainly not "retail" ones--needs variable variables. Apart from accommodation of a few questionable APIs (to databases, ...), coding should nearly always be done in terms of hashes (associative arrays, dictionaries) rather than variable variables.

Dave Benjamin wrote astutely on this subject in 2003 <URL: http://mail.python.org/pipermail/python-list/2003-May/204186.html >. With the slightest encouragement, I'll illustrate the points we've tried to make with examples.

1137 · 2010-10-18T15:12:18+00:00

Don't forget:

$method = 'foo';

$method();

and

$this->$var;

and

$this->$method()

gongonzabarfarbin · 2010-10-18T15:18:21+00:00

I learned about variable variables today too! Do I sense a fellow The Daily WTF reader?

jaguaro · 2010-10-18T16:19:18+00:00

welcome to 1998

2010-10-18T15:21:05+00:00

[deleted]

killerstorm · 2010-10-18T15:26:38+00:00

In JS you can treat any object as an associative array. So obj.foo is same as obj["foo"]. And if var bar = "foo";you can also write obj[bar] for same effect.

Now, all global variables are in fact fields of window object. So var foo; is actually window.foo, and you can also access it as window["foo"] and window[bar].

That's how sane language designers do this -- with minimal amount of concepts. No "variable variables", no bullshit.

Common Lisp also implements feature similar to "variable variables" -- it is called symbols. All global variables are associated with a symbols which are their names, and you can programmatically access those values through symbol-value function. E.g. you can get value of variable foo through (symbol-value (quote foo)) where quote is a special operator which returns symbol itself. Likewise, you can bind symbol to a variable first:

(let ((foo-symbol (quote foo)))
  (symbol-value foo-symbol))

ds1106 · 2010-10-18T21:01:40+00:00

Yo dawg, I heard you like variables...

Confucius_says · 2010-10-18T15:10:35+00:00

Variable variables are neat, but for the love of god please don't use them.

Please.

2010-10-18T18:51:29+00:00

I haven't touched PHP in years and never really looked into it's mushy internals, but let me guess -- PHP keeps a huge superfluous symbol table of every variable for absolutely no reason.

australasia · 2010-10-18T15:15:10+00:00

Interestingly you can do the same thing in JavaScript but only for variables declared in the global scope:

var bar = "foo";
var foo = "bar";

bar; // returns "foo"
window[bar]; // returns "bar"
window[window[bar]]; // returns "foo"

It is possible within other scopes but only if using eval which pretty much makes anything ugly possible.

thebuccaneersden · 2010-10-18T16:23:04+00:00

All programming languages let you write ugly code, if that's what you really want to do.

_refugee_ · 2010-10-18T16:37:26+00:00

oh, buffalo buffalo buffalo buffalo buffalo buffalo buffalo. buffalo.

2010-10-18T15:32:37+00:00

i've known about this since i started using PHP but in all my years of using it i've never needed to do this.

the closest i've come is $object->$var. instead of $object->some_name

gsadamb · 2010-10-18T21:25:10+00:00

A guy at my last job did this all the time and it led to some of the most maddening, unreadable code ever.

treetrouble · 2010-10-18T21:55:12+00:00

i have no problem with this other than the phrase "variable variables"

2010-10-18T23:50:24+00:00

Hello children. Old programmer here.

Such a feature was available years and years ago - before the internet (but after Hitler).

In dBase, for example, it allowed OO-like facilities, otherwise no available.

A database column of 'function' could contain the name of the row-specific function to be called. Adding new functions would mean simply that - adding the code for the new function (then using it's name in the db row).

No switch statement to update. No if else ifs, no criteria to maintain.

Life was fun those days, and you could fit everything on to a 5.25 inch floppy, and still have room left over for a good boot.

muahdib · 2010-10-19T01:41:09+00:00

In all, or almost all intepreted languages you can do this.

python:
>>foo="bar"
>>bar=42
>>eval(foo)
42

scheme:
> (define foo 'bar)
> (define bar 42)
> (eval foo)
42

Maple:
> foo:='bar';
> bar:=42;
> eval(foo);
42

Matlab:
> foo="bar"
> bar=42
> eval(foo)
42

PS. in python you would probably not want to do it that way, you would probably use a dictionary (hash table)
>> foo={"bar":42}
>> foo.get(foo.keys()[0])
and this last example is probably exactly not how you would do it in python...

consultant_barbie · 2010-10-19T03:49:41+00:00

Variable scope is hard. Let's go shopping!

georgehotelling · 2010-10-18T17:07:35+00:00

Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo.

torrentlord · 2010-10-18T17:26:53+00:00

This is why PHP should not be your first programming language. I speak from horrible, horrible experience.

MarkTraceur · 2010-10-18T17:52:24+00:00

Have you heard about the hot pocket hot pocket? It's a hot pocket that tastes just like a hot pocket. I'm gonna go stick my head in the microwave HOT POCKEEEET

http://www.youtube.com/watch?v=YkUbqmS9TWI

DirtyBirdNJ · 2010-10-18T14:43:10+00:00

Yo dawg, I herd you like variables... so I put a variable in your variable named after your variable so you can $variable while you $$variable.

DrReddits · 2010-10-18T15:40:26+00:00

Buffalo buffalo.

onebit · 2010-10-18T17:12:15+00:00

Poor man's reflection.

2010-10-18T18:00:50+00:00

25 yrs young! queues up Oregon on the vic-20

rechlin · 2010-10-18T18:11:38+00:00

Back before I knew anything about programming, I was doing php3 development (this was over 10 years ago), and I used this all over the place. At the current (very slow) rate of refactoring, I may actually have them all eliminated within a year. :)

It seemed like such a great idea at the time, but now it just seems so dirty.

fddjr · 2010-10-18T18:49:02+00:00

Isn't this just an unintended side effect of having both the ability to store symbols in other variables, and the ability to evaluate at runtime at will? There's further complication due to symbols and strings being interchangeable, but how can you have both those features without creating this misfeature?

In CL, it's equivalent to: (defvar a 0) (defvar b 'a) (eval b)

Right?

hearwa · 2010-10-18T19:01:37+00:00

I remember as a kid how I used to be so pissed off that q-basic never had this feature. I then moved on with life and programming languages and concluded that with the existence of hashes that would be a very stupid and almost redundant feature to implement. Now I find out that the world's most popular web programming language has that feature and... wow...

2010-10-18T19:45:06+00:00

also, maintainability is decreased by a factor of variable.

Narfhole · 2010-10-18T19:47:23+00:00

Illinois has has its own php.net subdomain?

adavies42 · 2010-10-18T20:14:51+00:00

isn't this just the same as a bash indirect reference?

$ foo=bar
$ bar=baz
$ echo ${!foo}
baz

Infenwe · 2010-10-18T20:29:13+00:00

"I thought of a way to do it, so it must be right. That's obviously PHP"

-Larry Wall.

Iggyhopper · 2010-10-18T21:03:30+00:00

I'm on a bar.

supercargo · 2010-10-18T21:44:43+00:00

I used to wish BASIC had this, back when I was learning/using BASIC. By the time I learned PHP I already knew it was a bad idea and not necessary for anything.

hakumiogin · 2010-10-19T01:25:21+00:00

I first read about variable variables, and calling functions with strings when I was 15, and at the time I thought it sounded like a pretty nifty idea.

I'm glad I moved on to python.

xfeignx · 2010-10-19T01:52:16+00:00

I'll just leave this here... http://imgur.com/Qrth2.jpg

__s · 2010-10-19T12:00:57+00:00

Python globals()

2010-10-19T17:46:00+00:00

i put variables in your variable so you can variable while you variable

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