Insert ID field generated from trigger, but not passed along - php

Question

In MySQL, I have a trigger:
BEGIN
IF (EXISTS(SELECT * FROM devices WHERE device_id = NEW.device_id)) THEN
SET NEW.id = NULL;
ELSE
INSERT INTO objects (object_type) VALUES ('3');
SET NEW.id = LAST_INSERT_ID();
END IF;
END
When this trigger gets a new id (from the objects table) it inserts the id into the id column of the devices table.
When I refer to it (for example with mysql_insert_id(); in PHP), its empty.
How can I return the insert id from the trigger (LAST_INSERT_ID();) to the function in PHP as the mysql_insert_id(); ?

Answer 1

Personally I use stored procedures.
Here is a basic example with PDO:
Code to create the Stored Procedures:
CREATE DEFINER=`user`#`localhost` PROCEDURE `InsertUser`(IN `Input_username` INT, OUT `Out_ID` INT)
LANGUAGE SQL
NOT DETERMINISTIC
CONTAINS SQL
SQL SECURITY DEFINER
COMMENT ''
BEGIN
INSERT INTO users(
username)
VALUES (
Input_username);
SET Out_ID = LAST_INSERT_ID();
SELECT Out_ID;
END
And PHP code:
$insert = "CALL InsertUser(:Input_username,
#Out_ID)";
$bdd = new PDO('mysql:host=localhost;dbname=db-name', 'user', 'password');
$stmt = $bdd->prepare($insert);
$stmt->bindParam(':Input_username', rand(), PDO::PARAM_STR); // to create random name
$stmt->execute();
$tabResultat = $stmt->fetch();
$id_user = $tabResultat['Out_ID'];
var_dump($id_user);
I hope I have helped. :)

Answer 2

This behaviour is by design:
If a stored procedure executes statements that change the value of LAST_INSERT_ID(), the changed value is seen by statements that follow the procedure call.
For stored functions and triggers that change the value, the value is restored when the function or trigger ends, so following statements will not see a changed value.
Workaround 1: Stored Procedures
Unfortunately this introduces a risk of inconsistencies between your table and objects, as insertions could still happen outside of this procedure (this problem could be adressed with convoluted access restrictions on the table)
Workaround 2:
Save the value in a user variable:
CREATE TRIGGER
....
BEGIN
INSERT INTO objects (object_type) VALUES ('3');
SET NEW.id = LAST_INSERT_ID();
SET #myLastInsertID = LAST_INSERT_ID();
END //
INSERT INTO your_table... -- trigger the above
SELECT #myLastInsertID; -- here is your value
Workaround 3:
Simply get the value from object ;)
INSERT INTO your_table... -- trigger the above
SELECT MAX(autoinc_column) FROM objects; -- here is your value!
Workarounds 2 and 3 should be wrapped in a transaction to ensure no-one interferes with #myLastInsertID or object during the process.