Session start proble [closed]

Question

I made a custom theme and it's working fine on my WAMP development server but when put on the LAMP staging environment I get this error from inside my functions.php file:

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent 

I've checked around and tried to remove the BOM from the beginning of the file but that didn't have any effect.

The code error message indicates that the problem is in this chunk of code:

<?php
function custom_excerpt_length( $length ) {
  return 20;
}
add_filter( 'excerpt_length', 'custom_excerpt_length', 999 );
?>

Any suggestions welcome.