Question

I am not able to get the lastinsertid value using $db->lastInsertId(); or $db->lastInsertId('fid');

$stmt = $db->stmt_init();   

$sql = "INSERT INTO ch_files_details (name,extension,size,parent) VALUES (?,?,?,?)";

$stmt = $db->prepare($sql) or die($db->error());

$stmt->bind_param('ssii', $filename, $extension, $filesize, $parent);  

$stmt->execute();

$fid = $db->lastInsertId();

Something missing or wrong there?

I am able to insert into the table but simply cannot get the last inserted id.

Table Definition: 
`ch_files_details` (
`fid` bigint(20) NOT NULL AUTO_INCREMENT,
`name` text NOT NULL,
`extension` text NOT NULL,
`size` bigint(20) NOT NULL,
`parent` bigint(20) NOT NULL,
PRIMARY KEY (`fid`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=26 ;

Answer 1

PDO::lastInsertId does not work here, because you are not using PDO. The code you have above is MySQLi code, not PDO. Use mysqli::$insert_id.

// This is all MySQLi prepared statement code, not PDO!!!
// PDO uses bindParam(), not bind_param(), doesn't have stmt_init(), and doesn't use the 'ssii' type strings for binding
$stmt = $db->stmt_init();   
$sql = "INSERT INTO ch_files_details (name,extension,size,parent) VALUES (?,?,?,?)";
$stmt = $db->prepare($sql) or die($db->error());
$stmt->bind_param('ssii', $filename, $extension, $filesize, $parent);  
$stmt->execute();

// insert_id is a property of the connection MySQLi object
$fid = $db->insert_id;

Review the MySQLi manual for full details. At some point you ended up in the wrong documentation set.