Lets say I have this:
function myFunc()
{
global $distinct_variable;
die ($distinct_variable);
}
function anotherFunc()
{
$distinct_variable = 'Hello World';
myFunc();
}
anotherFunc();
For anotherFunc() to correctly show 'Hello World', it must be written like this
{
global $distinct_variable;
$distinct_variable = 'Hello World';
myFunc();
}
Now it will show the message, but why must I global $distinct_variable; in anotherFunc() since it is a global in myFunc() which is within anotherFunc()
Yes, I know that variables inside functions won't go outside of them, but I was thinking it should have worked...
Could someone explain why isn't it working?
Thanks.
Thank you for your answers, I get it now :)
A global variable is exactly that - it exists in the GLOBAL scope ONLY.
Everything in PHP (except superglobals) exist in only one scope - be that the global scope, or the scope of a function/method. Scope does not cascade - so just because you have a variable in an "outer" function does not make it available to an "inner" function.
Similarly, global fetches the variables defined in the GLOBAL scope only (the top-most scope), not simply "the scope above this one, from which I was called". This is what you tried to do, but it absolutely will not work. This level of finer-grained control is what function arguments/return values are for.
Each function has its own symbol table. There is also a global symbol table. Just because one function is being called from within another doesn't mean that the variables declared global in one are global in the other, or inherited from the other. They still refer to the variable in the "local" symbol table by default.
Doing global $somevar; echo $somevar boils down to echo $GLOBALS['somevar'];. That $GLOBALS superglobal does not include variables that were defined inside a function: only truly 'global' vars, which exist at the top level of the script.
Related
I have heard using global variables is not good, however I am just trying to understand how the PHP language works. I am a beginner in the coding world.
Why can global variables be created within functions? Whether it is through the use of the global keyword or through a superglobal variable. I thought these two actions were used to access global variables in a function. I thought the only way you can create a global variable is to create it outside a function; in the global scope. I have looked at many different websites including w3schools.com and php.net
This is just some simple code I created to try and understand the way global variables work with functions:
<?php
function sample1() {
global $a;
echo $a = "this ";
}
sample1();
function sample2() {
echo $GLOBALS['$b'] = "is ";
}
sample2();
function sample3() {
global $c;
$c = "an ";
}
sample3();
echo $c;
function sample4() {
$GLOBALS['$d'] = "example ";
}
sample4();
echo $GLOBALS['$d'];
?>
This is the result of the code:
this is an example
All of the code works, but I don't understand how I created a global variable on any of these blocks of code? The global variables were not created outside of the functions. How can they be created inside of a function? What am I missing? Any response is appreciated - If possible, please keep the answer simple - I would like to discuss this further in the comment section, because I'm sure I will have follow up questions - Thank you
Variables can be created in the global scope in the two ways you just did - there's nothing saying that a function can't create (or change) a variable in the global scope - WHEN you explicitly ask for it through $GLOBALS or the global keyword.
The issue is that your belief "I thought the only way you can create a global variable is to create it outside a function; in the global scope." is not an exact statement. When you're using $GLOBALS and global, you're referring to the global scope. You're introducing a reference to the global scope inside your function.
With global you're in effect linking the local reference to the global reference, while with $GLOBALS you're explicitly referencing the global scope (which internally inside PHP can be introduced to the local scope in the same way).
In that case you're explicitly saying "I want this variable to be available in the global scope, make it so!" and PHP does what you're asking it to. This behaviour differ between languages, but as you've discovered for PHP, it's allowed.
It's not something I would recommend using in any way - it makes your code very hard to follow and argue around, so consider it an esoteric detail.
Simple question - I'm working with someone else's code and thus would break update-ability if I modify code outside what I'm working on. I'm just wondering in PHP if initialization is implied with the 'global' keyword inside of a function. i.e. is...
// some code
function myFunc() {
global $foo;
}
// other code
reasonably similar to....
// some code
$foo = ""; // any initialization value
function myFunc() {
global $foo;
}
// other code
I prefer to initialize variables in the base scope in which they are used, but as mentioned, that would require me to change the external code in this case.
The global statement merely states that when the current function refers to the variable, it accesses it in the global scope rather than the local scope. It doesn't have any effect on the variable's value by itself.
In other words, it tells PHP that any reference to $foo within that function should be treated equivalent to $GLOBALS['foo']. Assigning it will assign the global variable, and reading from it will access the global variable.
<?php
$foo = 1;
function meh(){
// <-- $foo can't be accessed
}
It doesn't look like a global variable. However, does it have disadvantages like global stuff if it's outside the function?
All variables defined outside any function are declared in global scope. If you want to access a global variable you have two choices:
Use the global keyword
<?php
$a = 1;
$b = 2;
function Sum()
{
global $a, $b;
$b = $a + $b;
}
?>
Or use the $GLOBALS
<?php
$a = 1;
$b = 2;
function Sum()
{
$GLOBALS['b'] = $GLOBALS['a'] + $GLOBALS['b'];
}
?>
Read more at http://php.net/manual/en/language.variables.scope.php
Yes. They can be accessed from any location, including other scripts. They are slightly better as you have to used the global keyword to access them from within a function, which gives more clarity as to where they are coming from and what they do.
The disadvantages of global variables apply, but this doesn't instantly make them evil as is often perceived in some OO languages. If they produce a good solution that's efficient and easily understandable, then you're fine. There are literally millions of succesful PHP projects that use global variables declared like this. The biggest mistake you can make is not using them and making your code even more complicated when it would have been perfectly fine to use them in the first place. :D
<?php
$foo = 1;
function meh(){
global $foo;
// <-- $foo now can be accessed
}
?>
Outside of the function is sorta like global scope (when compared to C-like languages), but you have to do one thing to allow access to the var within a function:
function meh(){
global $foo;
// $foo now exists in this scope
}
In your example $foo gets created as variable in the global scope. (Unless your shown script was included() from within another functions/methods scope.)
PHP doesn't have real global variables. You have to manually alias it using the global $foo; statement to access them. (Also the "anything global is bad" advise is just that, bad advise.)
If I understand your question correctly, there really shouldn't be a problem. Unless you declare a variable as a global, it will been limited to the scope in which it is declared, in this case whatever php file the above code is defined in. You could declare another variable $foo in meh() and it would be independent of the $foo defined outside.
I have a function in php:
function importSomething(){
include_once('something.php');
}
How do i make it sot that the include_once has a global effect? That everything imported will be included in the global scope?
You can return all the variables in the file like so...
function importSomething(){
return include_once 'something.php';
}
So long as something.php looks like...
<?php
return array(
'abc',
'def'
);
Which you could assign to a global variable...
$global = importSomething();
echo $global[0];
If you wanted to get really crazy, you could extract() all those array members into the scope (global in your case).
include() and friends are scope-restricted. You can't change the scope that the included content applies to unless you move the calls out of the function's scope.
I guess a workaround would be to return the filename from your function instead, and call it passing its result to include_once()...
function importSomething() {
return 'something.php';
}
include_once(importSomething());
It doesn't look as nice, and you can only return one at a time (unless you return an array of filenames, loop through it and call include_once() each time), but scoping is an issue with that language construct.
If you want ordinary variable definitions to be teleported into the global scope automatically, you could also try:
function importSomething(){
include_once('something.php');
$GLOBALS += get_defined_vars();
}
However, it if it's really just a single configuration array, I would also opt for the more explicit and reusable return method.
I know this answer is really late for this user, but this is my solution:
inside your function, simply declare any of the vars you need to ovewrite as global.
Example:
Need to have the $GLOBALS['text'] set to "yes":
Contents of index.php:
function setText()
{
global $text;
include("setup.php");
}
setText();
echo 'the value of $text is "'.$text.'"'; // output: the value of $text is "yes"
Contents of setup.php:
$text = "yes";
The solution is similar to mario's, however, only explicitely globally-declared vars are overwritten.
All variables brought in from an included file inherit current variable scope of the including line. Classes and functions take on global scope though so it depends what your importing.
http://uk.php.net/manual/en/function.include.php
(Final Paragraph before example)
I'm having trouble with global variables in php. I have a $screen var set in one file, which requires another file that calls an initSession() defined in yet another file. The initSession() declares global $screen and then processes $screen further down using the value set in the very first script.
How is this possible?
To make things more confusing, if you try to set $screen again then call the initSession(), it uses the value first used once again. The following code will describe the process. Could someone have a go at explaining this?
$screen = "list1.inc"; // From model.php
require "controller.php"; // From model.php
initSession(); // From controller.php
global $screen; // From Include.Session.inc
echo $screen; // prints "list1.inc" // From anywhere
$screen = "delete1.inc"; // From model2.php
require "controller2.php"
initSession();
global $screen;
echo $screen; // prints "list1.inc"
Update:
If I declare $screen global again just before requiring the second model, $screen is updated properly for the initSession() method. Strange.
Global DOES NOT make the variable global. I know it's tricky :-)
Global says that a local variable will be used as if it was a variable with a higher scope.
E.G :
<?php
$var = "test"; // this is accessible in all the rest of the code, even an included one
function foo2()
{
global $var;
echo $var; // this print "test"
$var = 'test2';
}
global $var; // this is totally useless, unless this file is included inside a class or function
function foo()
{
echo $var; // this print nothing, you are using a local var
$var = 'test3';
}
foo();
foo2();
echo $var; // this will print 'test2'
?>
Note that global vars are rarely a good idea. You can code 99.99999% of the time without them and your code is much easier to maintain if you don't have fuzzy scopes. Avoid global if you can.
global $foo doesn't mean "make this variable global, so that everyone can use it". global $foo means "within the scope of this function, use the global variable $foo".
I am assuming from your example that each time, you are referring to $screen from within a function. If so you will need to use global $screen in each function.
You need to put "global $screen" in every function that references it, not just at the top of each file.
If you have a lot of variables you want to access during a task which uses many functions, consider making a 'context' object to hold the stuff:
//We're doing "foo", and we need importantString and relevantObject to do it
$fooContext = new StdClass(); //StdClass is an empty class
$fooContext->importantString = "a very important string";
$fooContext->relevantObject = new RelevantObject();
doFoo($fooContext);
Now just pass this object as a parameter to all the functions. You won't need global variables, and your function signatures stay clean. It's also easy to later replace the empty StdClass with a class that actually has relevant methods in it.
You must declare a variable as global before define values for it.
The global scope spans included and required files, you don't need to use the global keyword unless using the variable from within a function. You could try using the $GLOBALS array instead.
It is useless till it is in the function or a class. Global means that you can use a variable in any part of program. So if the global is not contained in the function or a class there is no use of using Global