fwrite to a pre-defined location

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What I need is to write to a file about 30 bytes in. It's a login
scipt, and now what it does is fwrites a command at the end of a file,
but it needs to be midway through the file in order for it to work.
Preferably it needs to write to another file, then be read by
login.php. But that doesn't work because it doesn't know why there is
an if statement.

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