Note that unlike Java and C++, variables declared inside blocks such as loops or if's, will also be recognized and accessible outside of the block, so:
<?php
for($j=0; $j<3; $j++)
{
     if($j == 1)
        $a = 4;
}
echo $a;
?>

Would print 4.

Some interesting behavior (tested with PHP5), using the static-scope-keyword inside of class-methods.

<?php

class sample_class
{
  public function func_having_static_var($x = NULL)
  {
    static $var = 0;
    if ($x === NULL)
    { return $var; }
    $var = $x;
  }
}

$a = new sample_class();
$b = new sample_class();

echo $a->func_having_static_var()."\n";
echo $b->func_having_static_var()."\n";
// this will output (as expected):
//  0
//  0

$a->func_having_static_var(3);

echo $a->func_having_static_var()."\n";
echo $b->func_having_static_var()."\n";
// this will output:
//  3
//  3
// maybe you expected:
//  3
//  0

?>

One could expect "3 0" to be outputted, as you might think that $a->func_having_static_var(3); only alters the value of the static $var of the function "in" $a - but as the name says, these are class-methods. Having an object is just a collection of properties, the functions remain at the class. So if you declare a variable as static inside a function, it's static for the whole class and all of its instances, not for each object.

Maybe it's senseless to post that.. cause if you want to have the behaviour that I expected, you can simply use a variable of the object itself:

<?php
class sample_class
{ protected $var = 0; 
  function func($x = NULL)
  { $this->var = $x; }
} ?>

I believe that all normal-thinking people would never even try to make this work with the static-keyword, for those who try (like me), this note maybe helpfull.

Took me longer than I expected to figure this out, and thought others might find it useful.

I created a function (safeinclude), which I use to include files; it does processing before the file is actually included (determine full path, check it exists, etc).

Problem: Because the include was occurring inside the function, all of the variables inside the included file were inheriting the variable scope of the function; since the included files may or may not require global variables that are declared else where, it creates a problem.

Most places (including here) seem to address this issue by something such as:
<?php
//declare this before include
global $myVar;
//or declare this inside the include file
$nowglobal = $GLOBALS['myVar'];
?>

But, to make this work in this situation (where a standard PHP file is included within a function, being called from another PHP script; where it is important to have access to whatever global variables there may be)... it is not practical to employ the above method for EVERY variable in every PHP file being included by 'safeinclude', nor is it practical to staticly name every possible variable in the "global $this" approach. (namely because the code is modulized, and 'safeinclude' is meant to be generic)

My solution: Thus, to make all my global variables available to the files included with my safeinclude function, I had to add the following code to my safeinclude function (before variables are used or file is included)

<?php
foreach ($GLOBALS as $key => $val) { global $$key; }
?>

Thus, complete code looks something like the following (very basic model):

<?php
function safeinclude($filename)
{
    //This line takes all the global variables, and sets their scope within the function:
    foreach ($GLOBALS as $key => $val) { global $$key; }
    /* Pre-Processing here: validate filename input, determine full path
        of file, check that file exists, etc. This is obviously not
        necessary, but steps I found useful. */
    if ($exists==true) { include("$file"); }
    return $exists;
}
?>

In the above, 'exists' & 'file' are determined in the pre-processing. File is the full server path to the file, and exists is set to true if the file exists. This basic model can be expanded of course.  In my own, I added additional optional parameters so that I can call safeinclude to see if a file exists without actually including it (to take advantage of my path/etc preprocessing, verses just calling the file exists function).

Pretty simple approach that I could not find anywhere online; only other approach I could find was using PHP's eval().

In fact all variables represent pointers that hold address of memory area with data that was assigned to this variable. When you assign some variable value by reference you in fact write address of source variable to recepient variable. Same happens when you declare some variable as global in function, it receives same address as global variable outside of function. If you consider forementioned explanation it's obvious that mixing usage of same variable declared with keyword global and via superglobal array at the same time is very bad idea. In some cases they can point to different memory areas, giving you headache. Consider code below:

<?php

error_reporting(E_ALL);

$GLOB = 0;

function test_references() {
    global $GLOB; // get reference to global variable using keyword global, at this point local variable $GLOB points to same address as global variable $GLOB
    $test = 1; // declare some local var
    $GLOBALS['GLOB'] = &$test; // make global variable reference to this local variable using superglobal array, at this point global variable $GLOB points to new memory address, same as local variable $test

    $GLOB = 2; // set new value to global variable via earlier set local representation, write to old address

    echo "Value of global variable (via local representation set by keyword global): $GLOB <hr>";
    // check global variable via local representation => 2 (OK, got value that was just written to it, cause old address was used to get value) 

    echo "Value of global variable (via superglobal array GLOBALS): $GLOBALS[GLOB] <hr>";
    // check global variable using superglobal array => 1 (got value of local variable $test, new address was used)
    
    echo "Value ol local variable \$test: $test <hr>"; 
    // check local variable that was linked with global using superglobal array => 1 (its value was not affected)
    
    global $GLOB; // update reference to global variable using keyword global, at this point we update address that held in local variable $GLOB and it gets same address as local variable $test
    echo "Value of global variable (via updated local representation set by keyword global): $GLOB <hr>";
    // check global variable via local representation => 1 (also value of local variable $test, new address was used) 
}

test_references();
echo "Value of global variable outside of function: $GLOB <hr>";
// check global variable outside function => 1 (equal to value of local variable $test from function, global variable also points to new address)
?>

About more complex situation using global variables..

Let's say we have two files:
a.php
<?php 
    function a() { 
        include("b.php"); 
    }
    a();
?>

b.php
<?php
    $b = "something";
    function b() {
        global $b;
        $b = "something new";
    }
    b();
    echo $b;
?>

You could expect that this script will return "something new" but no, it will return "something". To make it working properly, you must add global keyword in $b definition, in above example it will be:

global $b;
$b = "something";

Note, as of 8.1 $GLOBALS is read only, so the example above writing a value into $GLOBALS no longer works.

Superglobal scope of class static properties, at the risk of stating the obvious.

<?php
class Alfa {
    public static $superglobal = 'visible everywhere';
}

function demoGlobal( ) {
    echo 'Alfa::$superglobal '.Alfa::$superglobal;
}

demoGlobal();   // outputs Alfa::$superglobal visible everywhere
?>