致命错误：调用未定义的函数session_is_registered（）

Question

A client asked me to solve this problem, but I don't understand anything.

--The error that appears--

Warning: mysql_connect(): The server requested authentication method unknown to the client [mysql_old_password] in /home/luipex/public_html/includes/adodb/drivers/adodb-mysql.inc.php on line 348

Warning: mysql_connect(): The server requested authentication method unknown to the client in /home/luipex/public_html/includes/adodb/drivers/adodb-mysql.inc.php on line 348

Warning: session_start(): Cannot send session cookie - headers already sent by (output started at /home/luipex/public_html/includes/adodb/drivers/adodb-mysql.inc.php:348) in /home/luipex/public_html/includes/ligacao.php on line 15

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/luipex/public_html/includes/adodb/drivers/adodb-mysql.inc.php:348) in /home/luipex/public_html/includes/ligacao.php on line 15

ligacao.php

session_start(); 15
$tempo_actual=time(); 16
if(!(session_is_registered("id_user_activo"))) 17
{ 18
$testesite=0; 19

---

adodb-mysql.inc.php

if (ADODB_PHPVER >= 0x4300) 346
        $this->_connectionID = mysql_connect($argHostname,$argUsername,$argPassword, 347
                                            $this->forceNewConnect,$this->clientFlags); 348
    else if (ADODB_PHPVER >= 0x4200) 349
        $this->_connectionID =  mysql_connect($argHostname,$argUsername,$argPassword, 350
                                            $this->forceNewConnect); 351