<?
 function xx() {
  global $a;
  unset($a);
 }

 // stage 1 - unset works
 $a=1;
 print "a= $a<br>";
 unset($a);
 print "a= $a<br>";
 // stage 2 - unset doesn't work in function xx
 $a=2;
 print "a= $a<br>";
 xx();
 print "a= $a<br>";

?>

My Result page:
a= 1
a= 
a= 2
a= 2  - it has to be empty because of previous unset

This script is working with PHP3.x