在 Code Igniter 框架中有几个这种行为的例子——在方法的开头创建一个变量，使用它，在方法结束之前，取消设置这个变量。
他们为什么要取消它们？重点是什么？据我所知，局部变量无论如何都会在方法结束时消失。
来自会话类的代码点火器（见最后几行）：
function sess_read()
{
// Fetch the cookie
$session = $this->CI->input->cookie($this->sess_cookie_name);
// No cookie? Goodbye cruel world!...
if ($session === FALSE)
{
log_message('debug', 'A session cookie was not found.');
return FALSE;
}
// Decrypt the cookie data
if ($this->sess_encrypt_cookie == TRUE)
{
$session = $this->CI->encrypt->decode($session);
}
else
{
// encryption was not used, so we need to check the md5 hash
$hash = substr($session, strlen($session)-32); // get last 32 chars
$session = substr($session, 0, strlen($session)-32);
// Does the md5 hash match? This is to prevent manipulation of session data in userspace
if ($hash !== md5($session.$this->encryption_key))
{
log_message('error', 'The session cookie data did not match what was expected. This could be a possible hacking attempt.');
$this->sess_destroy();
return FALSE;
}
}
// Unserialize the session array
$session = $this->_unserialize($session);
// Is the session data we unserialized an array with the correct format?
if ( ! is_array($session) OR ! isset($session['session_id']) OR ! isset($session['ip_address']) OR ! isset($session['user_agent']) OR ! isset($session['last_activity']))
{
$this->sess_destroy();
return FALSE;
}
// Is the session current?
if (($session['last_activity'] + $this->sess_expiration) < $this->now)
{
$this->sess_destroy();
return FALSE;
}
// Does the IP Match?
if ($this->sess_match_ip == TRUE AND $session['ip_address'] != $this->CI->input->ip_address())
{
$this->sess_destroy();
return FALSE;
}
// Does the User Agent Match?
if ($this->sess_match_useragent == TRUE AND trim($session['user_agent']) != trim(substr($this->CI->input->user_agent(), 0, 120)))
{
$this->sess_destroy();
return FALSE;
}
// Is there a corresponding session in the DB?
if ($this->sess_use_database === TRUE)
{
$this->CI->db->where('session_id', $session['session_id']);
if ($this->sess_match_ip == TRUE)
{
$this->CI->db->where('ip_address', $session['ip_address']);
}
if ($this->sess_match_useragent == TRUE)
{
$this->CI->db->where('user_agent', $session['user_agent']);
}
$query = $this->CI->db->get($this->sess_table_name);
// No result? Kill it!
if ($query->num_rows() == 0)
{
$this->sess_destroy();
return FALSE;
}
// Is there custom data? If so, add it to the main session array
$row = $query->row();
if (isset($row->user_data) AND $row->user_data != '')
{
$custom_data = $this->_unserialize($row->user_data);
if (is_array($custom_data))
{
foreach ($custom_data as $key => $val)
{
$session[$key] = $val;
}
}
}
}
// Session is valid!
$this->userdata = $session;
unset($session);
return TRUE;
}