UPDATE and PRIMARY KEY

Create this table :

CREATE TABLE `user_session` (
`php_id` varchar(26) NOT NULL,
`session_id` varchar(32) NOT NULL DEFAULT '',
`session_data` text,
`session_time` int(11) NOT NULL,
PRIMARY KEY (`php_id`,`session_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Add any values and try to update them :

UPDATE user_session SET session_id = :session_id, session_data = :session_data, session_time = :session_time WHERE php_id = :php_id

i get always this error :

SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 'xxxx' for key 'PRIMARY'

This is illogical, i try to update an existing line, i should not have this error.