Problem with accessing a global variable in PHP

Question

Following is code snippet :

function something() {
$include_file = 'test.php';
if ( file_exists($include_file) ) {
    require_once ($include_file);
//      global $flag;
//        echo 'in main global scope flag='.$flag;
    test();
   }
}

something();

exit;

 //in test.php

$flag = 4;
function test() {
   global $flag;

   echo '<br/>in test flag="'.$flag.'"';
   if ($flag) {
       echo 'flag works';
     //do something
   }
}

The above code snippet, echoes 'global scope' $flag value properly but doesnt recognises the $flag with value 4, assumes null value for $flag . Please point out what is wrong in accessing that $flag global variable.

Thanks in advance, Anitha

Answer 1

You are running in a problem here simply because the way PHP is currently interpreting your file. You have nested functions.

Here is how your code currently executes:

function something() {
    $include_file = 'test.php';
    if ( file_exists($include_file) ) {
        //require_once ($include_file);

        $flag = 4;
        function test() {
           global $flag;

           echo '<br/>in test flag="'.$flag.'"';
           if ($flag) {
               echo 'flag works';
             //do something
           }
        }

        //end require_once ($include_file);

        test();
    }
}

something();

exit;

As you can see, when you assign the value of 4 to $flag ($flag = 4), you are within the scope of the function something(), not within the global scope.

In test(), since you declare $flag as global within that function, $flag is a totally different variable, global to the whole script.

In order to avoid this problem, use the superglobal $GLOBALS. It's faster than using global anyway, and that way you don't get mingled in scoping issues as the ones above:

function something() {
    $include_file = 'test.php';
    if ( file_exists($include_file) ) {
        //require_once ($include_file);

        $GLOBALS['flag'] = 4;
        function test() {
           global $flag;

           echo '<br/>in test flag="'.$GLOBALS['flag'].'"';
           if ($GLOBALS['flag']) {
               echo 'flag works';
             //do something
           }
        }

        //end require_once ($include_file);

        test();
    }
}

something();

echo $flag; //echos 4

exit;

Answer 2

$flag = 4; is not in the global scope.

If the include occurs inside a function within the calling file, then all of the code contained in the called file will behave as though it had been defined inside that function.

-- PHP Manual page for include, which also applies for include_once, require, and require_once

I'm going to make a guess that the error you're getting is on the if ($flag) line, because at that point, $flag is uninitialized, because the global $flag variable has never been assigned a value.

Incidentally, echo 'global scope flag='.$flag; isn't displaying the global flag either, as you need a global $flag; in that function to display the global copy, which also has the side effect of making $flag = 4; affect the global copy in the included file.