Description:
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This is occurring in 5.4.0, although 5.4.x isn't an option under version.

Appears to be related to (or same as) 46569:

Calling fgets() after seek() to any non-zero line returns one line later than expected. seek(0) works fine and calling current() instead of fgets() also works correctly.

Test script:
---------------
Source file (temp.txt):
Line 1
Line 2
Line 3
Line 4

Code:
$f = new SplFileObject('temp.txt');
$f->seek(0);
echo $f->fgets();
$f->seek(1);
echo $f->fgets();
$f->seek(2);
echo $f->fgets();

Expected result:
----------------
Line 1
Line 2
Line 3

Actual result:
--------------
Line 1
Line 3
Line 4