Quick note when using next(), it appears that you have to already be at the end of the line in order for it to hop to the next one.  I realized this while attempting to do a lineCount implementaiton like the following:

<?php
  function lineCount($file)
  {
     $x=0;
     while(!$file->eof()) {
          $x++;
          $file->next();
     }
     return $x;
  }
  $file=new SplFileObject("something");
  echo lineCount($file);
?>

It ended up in an infinite loop.  The solution was to just call fgets()/current() in the loop, although it wasn't being used anywhere so the following works:

<?php
  function lineCount($file)
  {
     $x=0;
     while(!$file->eof()) {
          $file->current();
          $x++;
          $file->next();
     }
     return $x;
  }
  $file=new SplFileObject("something");
  echo lineCount($file);
?>

As @Jonnycake  pointed there is no documentation about the following behavior of  next();

You need to call current() to really move forward without the need of a source loop.

Be:
<?php
$file = new SplFileObject("file.txt");

echo PHP_EOL . $file->current();
$file->next();
 $file->next();
 $file->next();
echo PHP_EOL . $file->current(); // 2nd line of the file

?>

<?php
$file = new SplFileObject("file.txt");

echo PHP_EOL . $file->current();
$file->next(); $file->current();
$file->next(); $file->current();
$file->next(); 
echo PHP_EOL . $file->current(); // be the 4th line of the file

?>

Honestly, I don't know if it is  waste of memory and/or CPU .