PHP global in functions

Question

What is the utility of the global keyword?

Are there any reasons to prefer one method to another?

• Security?
• Performance?
• Anything els

Answer 1

I think everyone has pretty much expounded on the negative aspects of globals. So I will add the positives as well as instructions for proper use of globals:

1. The main purpose of globals was to share information between functions. back when there was nothing like a class, php code consisted of a bunch of functions. Sometimes you would need to share information between functions. Typically the global was used to do this with the risk of having data corrupted by making them global.

   Now before some happy go lucky simpleton starts a comment about dependency injection I would like to ask you how the user of a function like example get_post(1) would know all the dependencies of the function. Also consider that dependencies may differ from
   version to version and server to server. The main problem with dependency injection is dependencies have to be known beforehand. In a situation where this is not possible or unwanted global variables were the only way to do achieve this goal.

   Due to the creation of the class, now common functions can easily be grouped in a class and share data. Through implementations like Mediators even unrelated objects can share information. This is no longer necessary.

2. Another use for globals is for configuration purposes. Mostly at the beginning of a script before any autoloaders have been loaded, database connections made, etc.

   During the loading of resources, globals can be used to configure data (ie which database to use where library files are located, the url of the server etc). The best way to do this is by use of the define() function since these values wont change often and can easily be placed in a configuration file.

3. The final use for globals is to hold common data (ie CRLF, IMAGE_DIR, IMAGE_DIR_URL), human readable status flags (ie ITERATOR_IS_RECURSIVE). Here globals are used to store information that is meant to be used application wide allowing them to be changed and have those changes appear application wide.

4. The singleton pattern became popular in php during php4 when each instance of an object took up memory. The singleton helped to save ram by only allowing one instance of an object to be created. Before references even dependancy injection would have been a bad idea.

   The new php implementation of objects from PHP 5.4+ takes care of most of these problems you can safely pass objects around with little to no penalty any more. This is no longer necessary.

   Another use for singletons is the special instance where only one instance of an object must exist at a time, that instance might exist before / after script execution and that object is shared between different scripts / servers / languages etc. Here a singleton pattern solves the solution quite well.

So in conclusion if you are in position 1, 2 or 3 then using a global would be reasonable. However in other situations Method 1 should be used.

Feel free to update any other instances where globals should be used.