Create a Twitter url that enable users to Tweet an excerpt of your article using wordpress (and Advanced Custom Field if you wish so)

twitter-sharing-link.html

//Credits to Chris Coyier for this
//http://css-tricks.com/snippets/wordpress/automatic-social-media-links/
//http://css-tricks.com/video-screencasts/113-creating-and-using-a-custom-icon-font/

<a class="social-sharing" rel="nofollow" href="<?php echo tweet_this($post_id); ?>" title="Share this article with your Twitter followers">
    <i aria-hidden="true" class="social-icon twitter-icon"></i>
    <span>Tweet this !</span>
</a>

twitter_excerpt.php

function tweet_this($post_id){

//provide $post_id as argument to enable use out of the loop

//Fisrt get the fields if you provide different excerpts sizes :
    $long_exerpt    = get_field( "long_exerpt", $post_id );
    $short_exerpt   = get_field( "short_exerpt", $post_id );

//Define the max number of chars allowed in your Tweet
    $max_char        = 140;

//Test the different text sources to use as the excerpt you want to provide your user with in their tweets
  
  if ($short_exerpt){
    $excerpt = $short_exerpt;
  }
  elseif ($long_exerpt){
    $excerpt = $long_exerpt;
  }
  elseif (the_excerpt()){
    $excerpt = the_excerpt();
  }

//if nothing's found, use the title as a fallback
  else {
    $excerpt = get_the_title($post_id);
  }

//Start building the url

//Start the Tweet content
  $my_tweet  .= "Currently reading on example.com : ";

//Provide the Tweet with a link to your post
  $my_tweet  .= the_permalink();
  
// or $my_tweet  .= the_shortlink();
// or alternatively $my_tweet  .= site_url().'/?p='.$post_id;

//provide the Tweet with the excerpt created above
  $my_tweet  .= " " . $excerpt;

//Shorten the Tweet if needed
//$max_char-3 because you need to take the '...' at the end of the Tweet into account

  $my_tweet  = (strlen($my_tweet) > $max_char) ? substr($my_tweet,0,$max_char-3).'...' : $my_tweet;
  
  $my_tweet = urlencode($my_tweet);

//Create the URL
  $output  = 'http://twitter.com/home?status='. $my_tweet;

  return $output;
}