Stupid PHP Tricks: Illegal Variable Names

August 21, 2008 in PHP Tricks | 10 comments

From the PHP manual:

A valid variable name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: '[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*'

In other words, variable names can’t begin with a numeral. However there are a couple of ways to slip an illegal variable name into the symbol table:

$var = '1';
 
$$var = 'hello world';

or

${'1'} = 'hello world';

You can prove to your own satisfaction that the variables really exist with PHP’s rather obscure compact() function:

print_r( compact( '1' ));

Please note: if you use any of this in production code – and that includes compact() – I will come to your house and beat you with something heavy.

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  1. SeanJA’s avatar

    SeanJA on August 22, 2008 at 3:25 am

    I tried using compact once… It just screwed up my code, so I changed my mind about using it… good thing too apparently…

  2. Kia Kroas’s avatar

    Kia Kroas on August 22, 2008 at 5:14 am

    Instead of print_r(compact(…)); why don’t you use var_dump(); ?

  3. david’s avatar

    david on August 22, 2008 at 10:49 pm

    Yeah, I could have used var_dump($GLOBALS);, but then how would I have gotten a dig at compact() in? :)

  4. Mike Borozdin’s avatar

    Mike Borozdin on August 22, 2008 at 11:43 pm

    Funny :). But not very useful.

  5. Dav’s avatar

    Dav on August 23, 2008 at 9:46 am

    Dude, what’s wrong with compact()? Can be used to greatly clean up code in Frameworks like Cake. :o)

    $this->set(compact(‘data’, ‘moredata’, ‘evenmoredata’));

  6. Mads Skovbjerg Paldam’s avatar

    Mads Skovbjerg Paldam on August 24, 2008 at 7:57 pm

    This odd piece of apparently useless information makes for a nice trick-question in the “knowing absolutely everything there is to know about PHP quiz”… Thanks!

  7. Q’s avatar

    Q on August 25, 2008 at 2:24 pm

    oh dear… compact() *gah*

    it ranks up there or rather down there with eval()

    thanks for adding the following comment to ur post:

    “Please note: if you use any of this in production code – and that includes compact() – I will come to your house and beat you with something heavy.”

    I hope your readers take careful note… and the world cringes

  8. SayB’s avatar

    SayB on December 29, 2008 at 11:18 pm

    Hey Guys, I’ve noticed this weird behavior with php’s compact function. It simply does not work with variables that either have underscore in the name OR if they have one or more letters in caps or both. For example, $my_name won’t work, $MyName won’t work either but $myname works fine – anybody got a clue ?

  9. SayB’s avatar

    SayB on December 29, 2008 at 11:21 pm

    @Dav – FYI I say this after experimenting in Cake. It sure was swell not having to write all those dreaded $this->set() statements for each var :( – doh !