Question

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What happens if MySQL connections continually aren't closed on PHP pages?

If you forget sometimes to close the connection will be close in a while, automatically or will be stay open?

$link = mysql_connect(DB_HOST, DB_USER, DB_PASSWORD);
$db = mysql_select_db(DB_DATABASE);
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.
your code
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and you forget to mysql_close($link);

Answer 1

From mysql_connect() function documentation:

Note

The link to the server will be closed as soon as the execution of the script ends, unless it’s closed earlier by explicitly calling mysql_close().

PS. You should consider moving to PDO or at least mysqli driver.