<?php
$defaults = array(
'before' => '<p>' . __( 'Pages:' ),
'after' => '</p>',
'link_before' => '',
'link_after' => '',
'next_or_number' => 'number',
'separator' => ' ',
'nextpagelink' => __( 'Next page' ),
'previouspagelink' => __( 'Previous page' ),
'pagelink' => '%',
'echo' => 0
);
?>
You have to set the echo parameter to 0. Check the docs