How variable scope works

Avariable with global scope

      $a = 10; // $a has global scope
      function show_a() {
        echo("<p>Local: $a</p>\n");  // Inside function, $a is local,
                                     // so is not defined (NULL)
                                     // and displays an error if display_errors are turned on
      }
      show_a(); // Displays nothing

How to access a global variable from within a function

      $b = 10; // $b has global scope 

      function show_b() { 

        global $b; // Allow global variable $a to be accessed inside function

        echo("<p>\$b = $b</p>\n"); 

      } 

      show_b(); // Displays 10
      $b = 10

Another way to access a global variable from within a function

      $c = 10; // $c has global scope 

      function show_c() { 

        $c = $GLOBALS['c']; // $c now refers to a local variable named $c

        echo("<p>\$c = $c</p>\n"); 

      } 

      show_c(); // displays 10
      $c = 10

A variable with local scope function

      function show_d() { 
        $d = 10; // $d has global scope 
        echo("<p>inside functon \$d = $d</p>\n");
      }
      echo("<p>function $d = $d</p>\n"); // Outside function, $d is local,
                                         // so is not defined (NULL) 
                                         // and displays an error if display_errors are turned on

inside function $d = 10

$d = // Outside function, is local, and so is not defined (NULL) and displays an error if display_errors are turned on

Restrictions on the use of global variables

$GLOBALS['c'] = 15;    // Allowed – modifies element
$GLOBALS = [];         // NOT allowed – overwrites array
unset($GLOBALS);       // NOT allowed – deletes array
array_pop($GLOBALS);   // NOT allowed – removes element

The scope of a variable determines what code can access it. PHP only allows a variable to have a single scope. This means that a variable can only be used where it is defined.
If a variable is defined inside a function, it has local scope. Then, the variable is only available to code that runs inside that function and is not available outside the function or to any other functions.
If a variable is defined outside of a function, it has global scope. Then, by default, it is only available to code that runs at the global level and is not available within any functions.
To access a global variable from within a function, you can use the global statement to import a variable from global scope to local scope.
To get all variables that are stored in the global scope, you can use the built-in $GLOBALS array. This array is a superglobal variable like the $_POST and $_GET arrays.

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