use if($album_id == 0) { $album_id = NULL; } instead of if($album_id == 0) { $album_id == "NULL"; } .

"NULL" is a string. 

Remove the quotes and change ==(Checking equality) to =(Assigning a value to variable) in $album_id == "NULL";

use if($album_id == 0) { $album_id = NULL; } instead of if($album_id == 0) { $album_id == "NULL"; } .

"NULL" is a string. 

Remove the quotes and change ==(Checking equality) to =(Assigning a value to variable) in $album_id == "NULL";

$album_id = trim($_POST['album_id']); if($album_id == 0) { $album_id = NULL; } $genre_id = trim($_POST['genre_id']); if($genre_id == 0) { $genre_id = NULL; } $film_id = trim($_POST['film_id']); if($film_id == 0) { $film_id = NULL; } $category_id = trim($_POST['category_id']); if($category_id == 0) { $category_id = NULL; }