I have had a lot of problems with trying to destroy a session with session_destroy() as have many others I see, and none of the solutions brough up seem to work reliably.

What I found is that if I made any session-related functions (such as session_register etc) AFTER session_destroy(); in code, PHP returned an error.

This is regardless of whether session_destroy() was in another branch of a conditional statement:

e.g.

1 if(condition){
2   session_destroy();
3 {
4 else{
5   session_register("variable");
6 }

would return the error like "Warning: Cannot send session cache limiter - headers already sent (output started at /test.php:5) in /test.php on line 5)"

So even though session_register() would not be called if session_destroy was called and vice versa because of the branch in code, PHP still didn't like this.

The way to get around this is:

if(!condition){
    session_register("variable");
{
else{
    session_destroy();
}

Anyway, hopefully this will help you find the source of this frustrating problem so it can be fixed.