How to declare a global variable in php?

Question 1

I have code something like this:

<?
    $a="localhost";
    function body(){
        global $a;
        echo $a;
    }

    function head(){
        global $a;
        echo $a;
    }

    function footer(){
        global $a;
        echo $a;
    }
?>

is there any way to define the global variable in one place and make the variable $a accessible in all the functions at once? without making use of global $a; more?

Question 2

The $GLOBALS array can be used instead:

$GLOBALS['a'] = 'localhost';

function body(){

    echo $GLOBALS['a'];
}

From the Manual:

An associative array containing references to all variables which are currently defined in the global scope of the script. The variable names are the keys of the array.

If you have a set of functions that need some common variables, a class with properties may be a good choice instead of a global:

class MyTest
{
    protected $a;

    public function __construct($a)
    {
        $this->a = $a;
    }

    public function head()
    {
        echo $this->a;
    }

    public function footer()
    {
        echo $this->a;
    }
}

$a = 'localhost';
$obj = new MyTest($a);

Question 3

Well, why don't you just test ? ;-)

Note: It is not as easy as you'd think -- read the full answer ;-)

Calling the hello_testing(); function, without setting the variable:

hello_testing();

I get no output -- which indicates isset returned false.

Calling the function, after setting the variable:

$conditional_random = 'blah';
hello_testing();

I get an output:

foo is inside

Which indicates global works as expected, when the variable is set -- well, one should not have any doubt about that ^^

But note that isset will return false if a variable is set, and null!

See the manual page of isset()

Which means that a better test would be:

function hello_testing() {
    global $conditional_random;
    var_dump($conditional_random);
}

hello_testing();

And this displays:

null

No Notice: the variable exists! Even if null.

As I didn't set the variable outside of the function, it shows that global sets the variable -- but it doesn't put a value into it; which means it's null if not already set outside the function.

While:

function hello_testing() {
    //global $conditional_random;
    var_dump($conditional_random);
}

hello_testing();

Gives:

Notice: Undefined variable: conditional_random

It proves that notices are enabled ;-)

And, if global didn't "set" the variable, the previous example would have given the same notice.

And, finally:

function hello_testing() {
    global $conditional_random;
    var_dump($conditional_random);
}

$conditional_random = 'glop';
hello_testing();

Gives:

string 'glop' (length=4)

(This is to purely to demonstrate my example is not tricked ^^)

Question 4

Ideally, everything your function needs should be in a parameter.

function Sum($a, $b) {
  return $a + $b;
}

If you really can't avoid referring to variables outside your function scope, you have several options:

Use the $GLOBALS array:

function Sum() {
  return $GLOBALS['a'] + $GLOBALS['b'];
}

Use the global keyword:

function Sum()
  global $a, $b;
  return $a + $b;
}

Use an "anonymous" function and inject the globals

$Sum = function() use ($a, $b) {
  return $a + $b;
}

$result = $Sum();

Anonymous functions are more useful if you want to work with variables in-scope of the functional declaration that aren't global.

extract() them from $GLOBALS

function Sum() {
  extract($GLOBALS);
  return $a + $b;
}

This last option will pull in all globals, and seems to be what you're looking for.

But seriously, parametize whenever possible.

Question 5

Just define your variables in global.js outside a function scope:

// global.js
var global1 = "I'm a global!";
var global2 = "So am I!";

// other js-file
function testGlobal () {
    alert(global1);
}

To make sure that this works you have to include/link to global.js before you try to access any variables defined in that file:

<html>
    <head>
        <!-- Include global.js first -->
        <script src="/YOUR_PATH/global.js" type="text/javascript"></script>
        <!-- Now we can reference variables, objects, functions etc. 
             defined in global.js -->
        <script src="/YOUR_PATH/otherJsFile.js" type="text/javascript"></script>
    </head>
    [...]
</html>

You could, of course, link in the script tags just before the closing <body>-tag if you do not want the load of js-files to interrupt the initial page load.

Question 6

If you have to generate global variables in production code (which should be avoided) always declare them explicitly:

window.globalVar = "This is global!";

While it is possible to define a global variable by just omitting var (assuming there is no local variable of the same name), doing so generates an implicit global, which is a bad thing to do and would generate an error in strict mode.

How to declare a global variable in php?

Answers

php,global-variables,global