The documentation is not entirely clear when it comes to static variables. It says:
If a static variable is unset() inside of a function, unset() destroys the variable and all its references. 
<?php
function foo() 
{
 static $a;
 $a++;
 echo "$a\n";
 unset($a);
}
foo();
foo();
foo();
?> 
The above example would output: 
1
2
3
And it does! But the variable is NOT deleted, that's why the value keeps on increasing, otherwise the output would be:
1
1
1 
The references are destroyed within the function, this handeling is the same as with global variables, the difference is a static variable is a local variable.
Be carefull using unset and static values as the output may not be what you expect it to be. It appears to be impossible to destroy a static variable. You can only destroy the references within the current executing function, a successive static statement will restore the references.
The documentation would be better if it would say:
"If a static variable is unset() inside of a function, unset() destroys all references to the variable. "
Example: (tested PHP 4.3.7)
<?php
function foo() 
{
 static $a;
 $a++;
 echo "$a\n";
 unset($a);
 echo "$a\n";
 static $a; 
 echo "$a\n";
}
foo();
foo();
foo();
?> 
Would output:
1
1
2
2
3
3