Your trying to make sure that 'id' has been submitted and is a integer.
Whoever 'is_int((int)$_GET['id'])' doesn't do what you want. You need to use
'is_numeric($_GET['id'])'.
Do a test and you will see that 'is_int((int)$_GET['id'])' always return 1
(true).
Looking at your code in entries.php
'if ( isset($_GET['id']) && is_int((int)$_GET['id']) && isset($_GET['edit']) )'
Your trying to make sure that 'id' has been submitted and is a integer.
Whoever 'is_int((int)$_GET['id'])' doesn't do what you want. You need to use 'is_numeric($_GET['id'])'. Do a test and you will see that 'is_int((int)$_GET['id'])' always return 1 (true).