INOUT parameter failure for stored procedure called from prepared statement

Question

Here's my stored procedure:

DELIMITER $$

CREATE DEFINER=`root`@`localhost` PROCEDURE `testProc`(INOUT num INT(11), INOUT num2 INT(11))
BEGIN
    set num2 = num+7;
END

Here's the code that calls it:

$newId = 1;
$type - 2;

if ($stmt = mysqli_prepare($con, 'call testProc(?,?)')) {
    mysqli_stmt_bind_param($stmt, 'ii', $type,$newId);
    mysqli_stmt_execute($stmt);
    mysqli_stmt_bind_result($stmt, $type,$newId);
    echo $newId;
    exit;
    }

I expected $newId to hold 9. It still holds 1.

mysqli_stmt_bind_result() seems redundant, as I don't actually have a result set (and I think its existence causes bugs later because I don't have a result set), but this code falls over without it (My actual code doesn't but I don't know why). That's may be moot though.

Please can anyone show me how to change this code to make it work?

Answer 1

The PHP manual states that you've got to use session variables (MySQL sessions, not PHP)

INOUT/OUT parameter

The values of INOUT/OUT parameters are accessed using session variables.

mysqli_stmt_bind_result() is to be used to bind values from a resultset like a SELECT statement. This could be a SELECT statement in a stored procedure, see the chapter Stored Routine Syntax of the MySQL manual:

MySQL supports a very useful extension that enables the use of regular SELECT statements (that is, without using cursors or local variables) inside a stored procedure. The result set of such a query is simply sent directly to the client.

So you could do it with

// set your IN parameter using a prepared statement
if ($stmt = mysqli_prepare($con, 'SET @type := ?')) {
    mysqli_stmt_bind_param($stmt, 'i', $type);
    mysqli_stmt_execute($stmt);
}

// do the same for your second parameter 
if ($stmt = mysqli_prepare($con, 'SET @newId := ?')) {
    mysqli_stmt_bind_param($stmt, 'i', $newId);
    mysqli_stmt_execute($stmt);
}

// No need to use a prepared statement for the procedure call anymore
$result = mysqli_query($con, 'call testProc(@type, @newId)');
// If the procedure would return something else than the INOUT or OUT parameter
// then you would get this result now

// getting the value of the INOUT parameter
$result = mysqli_query($con, 'SELECT @newId as newId');
$row = mysqli_fetch_assoc($result);             
var_dump($row);

Output:

Connected to databasearray(1) {
  ["newId"]=>
  string(1) "9"
}

Note: Error handling left as an exercise to show only the main issue.