PHP, PDO & MYSQL : lastInsertId() returns NULL

Question

I am rather new at PDO-based MySQL and I'm running into a problem.

This is the method I'm executing :

public function insert( $table, $data )
{
  // utility functions to auto-format the statements
  $keys = $this->getKeys($data);
  $placeholders = $this->getPlaceholders($data);

  $q = "INSERT INTO $table ($keys) VALUES ($placeholders)";

  // this simply returns a new PDO object
  $dbh = $this->createSession();

  $stmt = $dbh->prepare($q);
  $stmt->execute( array_values($data) );

  return $dbh->lastInsertId();
}

After that, I run my method and store the returned value in a variable :

$new_user_id = $U->insert( $data );

var_dump($new_user_id);

And I get

NULL

Note the query is actually executed, and my data is correctly inserted into my table; no problem on that side. It seems it just can't grab the last insert ID as I ask for it.

Thanks for your time.

Answer 1

Not sure about any PDO-specific issues, but by default MySQL only returns an insert id if there's an auto_increment integer field in the database (generally but not necessarily the primary key). If your table doesn't include this nothing is returned by $dbh->lastInsertId()

Answer 2

I've reviewed the code again and I found that my value wasn't returned because of an intermediate method that wasn't passing the value correctly to the top-layer method.

Checking the value at the source shows no problem.

Thanks for the replies anyway.