Description:
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If you want to use negative fseek offset, it is not enough to use a negative offset as second parameter, also the third parameter must be specified as SEEK_END. The documentation should note that fact, currently it appears as if a function call like 

$i = fseek($filehandle, -100); 

would be adequate.

Reproduce code:
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http://php.net/fseek

Expected result:
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To move to a position before the end-of-file, you need to pass a negative value in offset and set the whence parameter to SEEK_END. 

Actual result:
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To move to a position before the end-of-file, you need to pass a negative value in offset .