// assumes we're on a 32-bit system

$fp = fopen($large_file, 'rb'); // assuming $large_file is 4GB or more
fseek($fp, PHP_INT_MAX, SEEK_SET);
echo ftell($fp), "\n"; // works as expected (displays value of PHP_INT_MAX)

fseek($fp, 1, SEEK_CUR); // call simply fails
echo ftell($fp), "\n"; // (displays value of PHP_INT_MAX)

fread($fp, 1); // works okay
echo ftell($fp), "\n"; // (displays value of -PHP_INT_MAX-1)
// (one would expect the previous fseek($fp, 1) call to do the same as the fread call, but anyway...)

fseek($fp, -1, SEEK_CUR); // works okay
echo ftell($fp), "\n"; // (displays value of PHP_INT_MAX)

fread($fp, 1);
fseek($fp, 1, SEEK_CUR); // works okay
echo ftell($fp), "\n"; // (displays value of -PHP_INT_MAX)

fseek($fp, -1, SEEK_CUR); // doesn't work! I'm guessing there's code to check if the file position would become negative, and deny the call if so
echo ftell($fp), "\n"; // (displays value of -PHP_INT_MAX)


// odd 8192 byte limitation to fseek'ing forward
fseek($fp, 8192, SEEK_CUR); // doesn't work!
echo ftell($fp), "\n"; // (displays value of -PHP_INT_MAX)

fseek($fp, 1, SEEK_CUR); // also fails!
echo ftell($fp), "\n"; // (displays value of -PHP_INT_MAX)

// restart everything
fseek($fp, PHP_INT_MAX, SEEK_SET);
fread($fp, 1);
fseek($fp, 8191, SEEK_CUR); // this works okay
echo ftell($fp), "\n"; // (displays value of -PHP_INT_MAX-1+8191)

fseek($fp, 1, SEEK_CUR); // doesn't work!
echo ftell($fp), "\n"; // (displays value of -PHP_INT_MAX-1+8191)

// it seems that there's some 8K buffer or similar with fseek - we need to "reset" this buffer with an fread to go further
fread($fp, 1);
fseek($fp, 8191, SEEK_CUR); // this now works
echo ftell($fp), "\n"; // (displays value of -PHP_INT_MAX-1+8191*2)