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I'm working on a PHP project, and from time to time between things I read online and things I see in forums, etc. I keep reading that you shouldn't use php globals. Making sure that I don't get that mixed up with PHP register_globals, because I'm not, I have been researching, but I haven't really found why or any type of alternatives.

So, my question is simple. Should I not use the global keyword in PHP? Additionally, if I shouldn't (or should), are there any alternatives? Reason being is, I have noticed that I need to access a variable defined in another file and I need to reference or call this variable in a function, lots of functions, and I'm kind of getting tired or using the global $var_name; code so much.

Any ideas (or am I just plain wrong)?

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    The "you should never use them" meme is not overly sensible advise. It very much depends on what you use them for. If you have more than 10 variables in the global/shared scope, then it's likely to be an issue. Please give a concrete example / list of your shared variables. Commented Aug 2, 2011 at 23:32
  • @mario: "If you have more than 10 variables in the global/shared scope, then it's likely to be an issue." Why? Is this a performance thing, or what? Commented May 26, 2012 at 14:31
  • @callum: No, certainly not a performance thingy. It just increases the likelihood of name clashes and misuse as trivial function signaling. (Not that signaling is bad per se). I just believe "less is more" regarding the shared scope. Commented May 26, 2012 at 18:46
  • OK, I agree the risk of name clashes goes up as the number of global variables goes up. But I don't think it's wise to think of any particular number as a rule of thumb. The risk varies based on so many other factors, like whether or not you prefix your global names with something unique to your app, and how big your codebase is. 100 globals in a single project, all carefully prefixed and documented, would be far safer than even one stray global with a generic name in another project. Commented May 26, 2012 at 19:10

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Static classes and singletons are only little better than globals. Static classes merely group global variables, but the variables themselves are still globally reachable, single instance variables. Same goes for Singletons. While they have their uses, they should not be used as a general replacement for globals. PHP makes it tempting though, especially because of having to declare global variables in functions, while static classes are available always and everywhere.

You'd best put the variables in a class (like a 'AppConfig' or more specific class), and create an instance of that class to hold specific values. Then, pass that instance to all methods in your framework. That way, you don't rely on a specific Singleton implementation, and are truly flexible.

But, I must admit that it's a lot of work, especially when you're not experienced yet. So using a singleton now is probably ok, as long as you keep this answer in mind when you feel your singletons itching somewhere in the future.

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You're not wrong, you just need to think of some architecture for your application. : )

If you have shared data between classes, you should use a model that contains that shared data and has an API accessible to all your classes to retrieve that variable.

For simplicity's sake, you can use a Singleton to contain any shared data.

The PHP Patterns Page has an example of a Singleton. The idea behind a Singleton is that you always access the same instance (version) of that class, so that if you change the variable there, it will automatically be changed elsewhere.

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Globals are only bad if you're writing Object Oriented Code. If you're code is procedural, globals are just fine. If its using objects, you need to use dependency injection (new Object(new Collaborator)). To manage that, you would probably need to use a dependency injection container eventually. If you start using static classes and singletons, you're actually no longer writing 100% OO code.

But bottom line, if its regular procedural code, there's nothing wrong with globals.

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I don't know why you got two down votes; everything you've written looks correct to me, and also answering the original question.
because he insinuated that globals are ok and he didn't shun procedural programming.
The danger of globals is that when your data can be changed from anywhere in the code, it's impossible to predict what its value will be at any given point. This is true in procedural programming just as in object-oriented programming.
Hardly. Procedural/imparative programming does not necessitate a global state. There is little difference in how much it can be modularized and utilize local scopes, effectively using it in a functional fashion.
@blockhead. That's why they invented parameters. I think they've been around since 1854. They allow you to pass a value to a procedure which doesn't have to be a global. In fact, you can get away with writing a procedural program that doesn't need any globals at all. Really! :)
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As some other's have said, there is nothing wrong with using globals; especially on a short script it can actually keep your code more readable than obscuring it with too much OO structure.

But you wrote: "I need [this variable in] lots of functions, and I'm kind of getting tired or using the global $var_name; code so much"

This suggests what you actually want is for all those functions using $var_name to be in a class. As a first stage refactor, you'll pass in the variable from the other file to the constructor, and replace all your $var_name references with $this->var_name, and then strip out all the global $var_name; lines.

You may end up with one global instance of this class, but that is okay. Globals aren't evil, but they should be managed and documented carefully as your code gets more complex.

If you don't already have it, Martin Fowler's Refactoring book is a good read to help you cope when your 100-line script is now 1000-lines and getting you down. (Examples are in java, but still accessible for a PHP programmer.)

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In general, global variables introduce problem with security and maintainability of code. If you are using modern PHP, then a good solution is to have a static class that can act as a holder for all the globals you need. For example, sfConfig from Symfony Framework is a good example. You can see the docs and code here for inspiration on how to make your own (or just clean up the code to use in your own project... it's fairly standalone).

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BTW, what I am suggesting is an example of the singleton pattern.

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