Sessão não funciona!

quando eu executo o comando session_start()da os seguintes erros, por que?Warning: session_start(): open(/tmp\sess_b18589b6cd3f2731f88a13508df000e1, O_RDWR) failed: No such file or directory (2) in c:\www\php\index.php on line 5Warning: session_start(): Cannot send session cookie - headers already sent by (output started at c:\www\php\index.php:4) in c:\www\php\index.php on line 5Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at c:\www\php\index.php:4) in c:\www\php\index.php on line 5Warning: mysql_connect(): Can't connect to MySQL server on 'localhost' (10061) in c:\www\php\index.php on line 6Warning: mysql_select_db(): supplied argument is not a valid MySQL-Link resource in c:\www\php\index.php on line 7Warning: Unknown(): open(/tmp\sess_b18589b6cd3f2731f88a13508df000e1, O_RDWR) failed: No such file or directory (2) in Unknown on line 0Warning: Unknown(): Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/tmp) in Unknown on line 0