“unset”函数在其他行之前调用

My question may look weired! "unset" function is called before its line. My example code is:

session_start();
print_r($_SESSION[$session_key]);
unset($_SESSION[$session_key][$key]);

this code give no result, and the session give empty. But when i comment the unset function, it show the session values normal:

session_start();
print_r($_SESSION[$session_key]);
//unset($_SESSION[$session_key][$key]);

please can someone help why this occur?