more.php

<?php 
function multiline_with_more($multiline_name){
        global $post;
        ///Get the content  of the multiline custom field
        $multiline =  get('multiline');
        //if the page is_Single  we replace the <!--more--> tag for:
        //<span id="more-9-{$custom_Field_name"></span> 
        if(is_single($post->ID)){
                preg_match('/(.*)(\<\!\-\-more\-\-\>)(.*)/is',$multiline,$matches);
                return $matches[1]."<span id='more-{$post->ID}-{$multiline_name}'></span>".$matches[3];

        }else{ 
                //if not is_single the page we put a link with the message "Read the rest of this entry »"
                //and remove all the content after to <!--more--> 

                $permalink = get_permalink( $post->ID ); 

                 preg_match('/(.*)(\<\!\-\-more\-\-\>)(.*)/is',$multiline,$matches);
                 return $matches[1]."<a href='{$permalink}#more-{$post->ID}-{$multiline_name}'>Read the rest of this entry</a>";
        }
}
?>