These two sources are the same except the last line of the second one.

The former represents that it doesn't work to unset the global variable,
the latter that it only works when unset throught the $GLOBALS variable.

[SRC 1: unset doesn't work]
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<?php
function unset_variable ($name, $flag)
  {
    global $$name;
    global $GLOBALS;

    if ($flag & 1)
      {
        echo "  - <font color=magenta>unset \$$name</font><br>\n";
        unset($$name);
      }
    if ($flag & 2)
      {
        echo "  - <font color=magenta>unset \$GLOBALS[\"$name\"]</font><br>\n";
        unset($GLOBALS[$name]);
      }
  }

function disp_variable ($name)
  {
    global $$name;
    global $GLOBALS;

    printf("  - \$%s : %s<BR>\n",
        $name, isset($$name) ? "<font color=green>".$$name."</font>" : "<font color=red>(unset)</font>");
    printf("  - \$GLOBALS[%s] : %s<BR>\n",
        $name, isset($GLOBALS[$name]) ? "<font color=green>".$GLOBALS[$name]."</font>" : "<font color=red>(unset)</font>");
  }

for ($i=0; $i < 4; $i++)
  {
    $name = "var$i";
    $$name = "VALUE_$i";
    $step = $i;
    $flag = $i;

    printf("<font color=blue>STEP (%s)</font><BR>\n", $step);

    disp_variable ($name);
    unset_variable ($name, $flag);
    disp_variable ($name);

    printf("<BR>\n");
  }
?>
============================================================

[SRC 2: unset works only throught the $GLOBALS variable]
============================================================
<?php
function unset_variable ($name, $flag)
  {
    global $$name;
    global $GLOBALS;

    if ($flag & 1)
      {
        echo "  - <font color=magenta>unset \$$name</font><br>\n";
        unset($$name);
      }
    if ($flag & 2)
      {
        echo "  - <font color=magenta>unset \$GLOBALS[\"$name\"]</font><br>\n";
        unset($GLOBALS[$name]);
      }
  }

function disp_variable ($name)
  {
    global $$name;
    global $GLOBALS;

    printf("  - \$%s : %s<BR>\n",
        $name, isset($$name) ? "<font color=green>".$$name."</font>" : "<font color=red>(unset)</font>");
    printf("  - \$GLOBALS[%s] : %s<BR>\n",
        $name, isset($GLOBALS[$name]) ? "<font color=green>".$GLOBALS[$name]."</font>" : "<font color=red>(unset)</font>");
  }

for ($i=0; $i < 4; $i++)
  {
    $name = "var$i";
    $$name = "VALUE_$i";
    $step = $i;
    $flag = $i;

    printf("<font color=blue>STEP (%s)</font><BR>\n", $step);

    disp_variable ($name);
    unset_variable ($name, $flag);
    disp_variable ($name);

    printf("<BR>\n");
  }
$GLOBALS[anything];
?>
============================================================