Description:
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I recently noticed that "\{$var}" retains the backslash. Older versions of php don't do so, because backslash is meant to escape the curly.

Reproduce code:
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<?php
        $length = 100;
        $sent_folder = "INBOX.Sent";

        echo "APPEND \"$sent_folder\" (\\Seen) \{$length}\r\n";
?>


Expected result:
----------------
APPEND "INBOX.Sent" (\Seen) {100}

Actual result:
--------------
APPEND "INBOX.Sent" (\Seen) \{100}