Global Variable Scope

Anon

I'm not looking for help here, but maybe someone in the know could answer a question that I've been wondering about for a while.

Why is it that in PHP, global variables are the opposite to other programming languages? The idea of a global variable is to make a variable that is accessable by the whole program. In PHP, you have to explicitly declare global variables inside of functions. Seems backwards to me. Gloabals should automatically be visable everywhere.

The only reason that I can think of is
because PHP is being used by fairly large group of people with varying levels of knowledge, and that this framework is in place to help prevent people who don't understand variable scoping (and the problems that it can present, espcially from delaclare all of your variables to be global) from writing sloppy code with all variables being declared as globals.

Brad.

Anon

Actually having variables that are automaticaly global is not a good idea. You want the programmers to know what variable they are using when they use it.

But that's neither here or there. PHP is built the way it is, I believe, because it's based on C, and coded in C which uses similar variable scoping rules.

toma42

Due in php 4.0.7:
- Introduced $GET, $POST, $COOKIE, $SERVER and $ENV variables, which
deprecate the old $HTTP*_VARS arrays. In addition to be much shorter to
type - these variables are also available regardless of the scope, and
there's no need to import them using the 'global' statement. (Andi & Zeev)

Anon

Imagine we can set variable as global for the whole php file. Then when the php interpreter examines this file it would take much more memory units to parse this variable for all functions modules and. Thus this file would take much more time for executing, even more - if the value of the variable is rather large the time for execution will increase several times.
I think in php it is released in the most appropriate way to have this variable parsed only in the function you are going to use!

Segundo

Anon

Actually in c if you declare mvyar as type int at the beginnnig of your program, that variable is visible throughout all of your code and functions. If you declare a variable inside of a function then that variable is only visible inside that function.

"The lexical scope of an object or function identifier in an external declaration begins at the end of its declarator and persits to the end of the transaltion unit in which it apears. The scope of a parameter of a function definition begins at the start of the block defining the function, and persists through the function; the scope of a parameter in a function declaration ends at the end of the declarator. The scope of an identifier declared at the head of a block begins at the end of its declarator, and persists to the end of the block."

And now for the bread and butter...

"The scope of a structure, union, or enumeration tag, or an enumeration constant, begins at its appearance in a type specifier, and persists to the end of the translation unit(for declarations at the external level) or to the end of the block (for declarations within a function). If an identifier is explicitly declared at the head of a block, including the block constituting a function, andy declaration of the identifier outside the block is suspended until the end of the block" - The C programming Language by: Dennis M.Ritchie (creator of C)

In simple terms all variables decalred outside of a function are visible inside all functions. Variables inside of functions are visible only inside of those functions. If there exists a variable X outside of a function then any varibale X inside of a function has precedence and will be used instead of the outside X.

The quote above of course goes even deeper then that since it deals with header files etc.. but this is not the topic of this discussion.

Anon

variables exist in memory only once, no matter if they are global or inside functions. Every variable has a memory pointer that gives it location. Since PHP is a interpreted language and not compiled like c then all variables are dynamically created. When you create a variable joe and set joe="hi" you asign space for that variable name and the linker links the variable name to the memory region that it occupies based on its type.(i.e. integers have a ficed memnory space and so do floats, but arrays and strings have variable memory space). When you have a global variable that pointer is available everywhere in your program, and moving a pointer around wastes no memory space at all. in short making these variables global will not waste alot of memory.

Anon

You are correct, but since any PHP code not in a function is assumed to be in the equivalent of a main() function in C then the variables declared there aren't global.