PHP variable variables

gistfile1.txt

<?php
# Reference http://us.php.net/language.variables.variable

class foo { static function hello() { print __LINE__." ".get_class()." world\n"; } }
class bar { function hello() { print __LINE__." ".get_class()." world\n"; } }
function hello() { print __LINE__." world\n"; }

call_user_func('hello');
call_user_func( array('foo','hello') );

$z = new bar();
call_user_func( array($z,'hello') );

$a = 'hello';
call_user_func($a);

$hello = 'world';
print __LINE__." ".$$a."\n";