Clarifying example:

<?php 

class foo {
	static $foo = array();
	static function test($obj) {
		self::$foo[] = $obj;
	}
}

class bar {
	public function __construct() {
		foo::test($this);
	}
}

$a = new bar;
unset($a);
var_dump(foo::$foo); // He expects empty array

?>

Documentation says:
"When assigning an already created instance of a class to a new variable, the new variable will access the same instance as the object that was assigned. This behaviour is the same when passing instances to a function."

Using PHP 5.3.0-dev (cli) (built: Dec 17 2007 10:53:40):

<?php

$obj = new stdClass;

$a = $obj;

unset($obj);

var_dump($a, $obj);

?>

Output:

object(stdClass)#1 (0) {
}
NULL

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

unset($a) or $a = NULL; only affects the current variable, unless this variable represents the last one holding a reference to the object, the object won't be destroyed.

For example:

$a = new StdClass;
$b = $a;
$a = NULL; // $b is still holding the instance.

same with unset:

$a = new stdClass;
$b = $a;
unset($a); // $b is still holding the var

You could have the object destroyed by $a = NULL; if $b was a reference to $a. but in case of arrays the array element is not removed: you should do such cleaning manually.
The behavior you notice is expected and has nothing specific to do with "static arrays".