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(PHP 5 >= 5.1.0)
SplFileObject::fseek — Seek to a position
$offset 
   [,  int $whence  = SEEK_SET 
  ] )
   Seek to a position in the file measured in bytes
   from the beginning of the file, obtained by adding
   offset to the position specified by
   whence.
  
offset 
       The offset. A negative value can be used to move backwards through the file which
       is useful when SEEK_END is used as the whence value.
      
whence  whence values are:
       
SEEK_SET  - Set position equal to offset bytes.SEEK_CUR  - Set position to current location plus offset.SEEK_END  - Set position to end-of-file plus offset.
       If whence is not specified, it is assumed to be  SEEK_SET .
      
Returns 0 if the seek was successful, -1 otherwise. Note that seeking past EOF is not considered an error.
Example #1 SplFileObject::fseek() example
  <?php
$file  = new  SplFileInfo ( "somefile.txt" );
 // Read first line
 $data  =  $file -> fgets ();
 // Move back to the beginning of the file
// Same as $file->rewind();
 $file -> fseek ( 0 );
 ?>   
[#1] wangbuying at gmail dot com [2013-08-28 11:25:35]
make sure you know this fseek in SplFileObject is different from \fseek.
Argument for fseek in SplFileObject is the line number, not the bytes.
Example:
<?php
$fp = new SplFileObject('./fseek.txt', 'rb');
$fp->fseek(70); // now go to line 70, not byte 70
echo $fp->fgets();
?>