PHP - foreach affects session value

Question

How can foreach loop affect session variable?

session_start();
$_SESSION[test] = "Session content";
echo $_SESSION[test].'<br />';

$test_array = array("test", "array", "something", "array end");

foreach($test_array as $test){
    echo $test.'<br />';
}

echo '<br />Session content after foreach: '.$_SESSION[test].'<br />';

When I run this code on some web hostings, its output is OK.

Session content
test
array
something
array end

Session content after foreach: Session content

But only at first execution (when session is created). When I execute this code second time (session is already created) its output looks like this:

Session content 
test 
array 
something
array end

Session content after foreach: array end

I don't know how can variable $test affect $_SESSION[test].

Answer 1

I'd bet you're using register globals and that means that if you have a session variable named test it will become a global variable named $test when you execute session_start(). Your loop then changes the value of $test, which is a global reference to the session variable.

See Using Register Globals and the register_globals directive.

Basically this is a good lesson why you shouldn't use register globals. In this case the name clash is probably harmless but you can potentially create huge problems this way, even vulnerabilities to attacks.