PHP programming forum. Ask questions or help people concerning PHP code. Don't understand a function? Need help implementing a class? Don't understand a class? Here is where to ask. Remember to do your homework!
Global variables are normally a maintence nightmare and can cause hard to diagnose bugs to appear. However, I don't think I would say there are any security concerns directly tied to them.
(This is assuming that register_globals is off and you havn't used a "trick" to simulate it being on.)
But the more variables are made global, the more memory is required. If you're doing a big project, or something that will be accessed very often, there are some obvious downsides to global variables.
Whoa, really? I always avoided them, because I heard that the larger a variable's scope is, the more memory it takes up. Huh, you learn something new every day
Globals are normally a sign of poor design/decomposition. Globals defeat encapsulatios and abstraction. They typically cause increase maintainence problems.
There are occasions when they are appropriate; however, they are used far too often because of laziness.