Description:
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when closing the connection to the db, and using 
properties of the $mysqli object (such as insert_id), PHP 
crashes (or even the webserver, I don't know) 
-> although closing it before is not the right way, at 
least an error message should be displayed 
 
(took me a while to recognize that 1. some properties are 
not accessible after disconnecting and 2. that this was 
the reason for not showing the whole page) 

Reproduce code:
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// taken from http://de2.php.net/manual/en/function.mysqli-insert-id.php

$mysqli = new mysqli('HOST', 'USER', 'PASSWORD','DB');
$mysqli->query("CREATE TABLE myCity LIKE City");
$query = "INSERT INTO myCity VALUES (NULL, 'Stuttgart', 'DEU', 'Stuttgart', 617000)";
$mysqli->query($query);

/* close connection */
$mysqli->close(); 

printf ("New Record has id %d.\n", $mysqli->insert_id);

Expected result:
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print out "New Record has id <id>" 

Actual result:
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"connection to <server> broken" error message from the 
browser