Description:
------------
symlink improperly creates a recursive link on existing symlink without a target.  This appears to be a regression since php 5.2 which was broken in php 5.3 and all subsequent versions.  In php 5.2, calling symlink on an existing symlink always failed regardless whether the target existed.

Test script:
---------------
<?php
/* 
Make symlink create a symbolic link that does not link to anything (link does not exist)
*/
symlink('43', '0'); // Result: 0 -> 43

// second call to the same thing
symlink('43', '0'); // Result: 43 -> 43

/* 
third call will fail with a warning:

PHP Warning:  symlink(): No such file or directory in test.php on line 2
*/
symlink('43', '0'); 

Expected result:
----------------
symlink should fail and emit a warning after the second call to a non-existing link and not create a recursive link on the target.

Actual result:
--------------
symlink fails on the third call to create a symlink that already exists after creating a recursive symlink to the target.