Automatic link checking

Why there’s a need

Link rot is one of those terrible curses of the Internet. You link to a page and two months later it’s gone. But you, as the author of your website, don’t know this. Only your visitors who click that link know. And most of them probably won’t bother to send you an email to tell you the link is broken.

But what if things weren’t like that? What if broken links never got displayed on your pages in the first place?

Well this first attempt here doesn’t do quite that much. Instead, it simply replaces bad links with their Google cache equivalents.

Notes

The code below was designed to work with PHP-Markdown; indeed, parts of it were blatantly lifted from Michel Fortin’s code. Why reinvent the wheel when you can steal someone’s car? It also requires that you have curl installed in order for it to work. To use, simply call Linkcheck() on your text before you call Markdown().

Code

<?php

#
# Linkcheck  -  A link checking tool for Markdown
#
# Copyright (c) 2004 Frank Mitchell
# <http://frankmitchell.org/2004/12/check-links>
#
# Based on Michel Fortin's port of John Gruber's Markdown.
# Those projects are copyright to their respective owners
# and can be found at the URLs below.
#
# PHP port of Markdown by Michel Fortin
# <http://www.michelf.com/projects/php-markdown/>
#
# Markdown by John Gruber
# <http://daringfireball.net/projects/markdown>
#

function Linkcheck($text)
{
  # This is the same as Michel Fortin's PHP code for Markdown.
  # Likewise, the link matching code was simply lifted from there.
  $md_nested_brackets_depth = 6;
  $md_nested_brackets =
    str_repeat('(?>[^\[\]]+|\[', $md_nested_brackets_depth).
    str_repeat('\])*', $md_nested_brackets_depth);
  $md_tab_width = 4;
  $less_than_tab = $md_tab_width - 1;

  # Grep the text for a list of links to check.
  # We need to handle inline links and reference links.

  # Get all the inline links.
  preg_match_all("{
    (                          # wrap whole match in $1
      \\[
        ($md_nested_brackets)  # link text = $2
      \\]
      \\(                      # literal paren
        [ \\t]*
        <?(.*?)>?              # href = $3
        [ \\t]*
        (                      # $4
          (['\"])              # quote char = $5
          (.*?)                # Title = $6
          \\5                  # matching quote
        )?                     # title is optional
      \\)
    )
    }xs", $text, $inline);

  # Get all the reference links.
  preg_match_all('{
    ^[ ]{0,'.$less_than_tab.'}\[(.+)\]:  # id = $1
      [ \t]*
      \n?                                # maybe *one* newline
      [ \t]*
    <?(\S+?)>?                           # url = $2
      [ \t]*
      \n?                                # maybe one newline
      [ \t]*
    (?:
      (?<=\s)                            # lookbehind for whitespace
      ["(]
      (.+?)                              # title = $3
      [")]
      [ \t]*
    )?                                   # title is optional
    (?:\n+|\Z)
    }xm', $text, $reference);

  # Merge our two lists of links into one array,
  # sort it, and remove dupliates.
  $links = array_merge($inline[3], $reference[2]);
  $links = array_unique($links);
  sort($links);

  # Build or curl command to check links.
  $command = 'curl -I';

  # We're not going to bother checking internal links now.
  # Let's just check external ones.
  for($i = 0; $i < count($links); $i++)
  {
    $link = $links[$i];
    if(substr($link, 0, 7) == 'http://')
    {
      $command .= ' '.$link;
      $checked_links[] = $link;
    }
  }

  # Run curl to check for bad links.
  exec($command, $output);

  # Process our results. The first line is going to be the status.
  # Blank lines will separate status calls.

  # What's our first result?
  $link_ptr = 0;
  if(strpos($output[0], '200') === false)
  {
    $bad_links[] = $checked_links[$link_ptr];
  }
  $link_ptr++;

  # Continue getting the rest of our results.
  for($i = 1; $i < count($output); $i++)
  {
    if($output[$i] == '')
    {
      if(strpos($output[$i + 1], '200') === false)
      {
        $bad_links[] = $checked_links[$link_ptr];
      }
      $link_ptr++;
    }
  }

  # Replace any bad links we found with Google's cached copies.
  $google_links = str_replace(
    'http://',
    'http://google.com/search?q=cache:',
    $bad_links);
  $text = str_replace($bad_links, $google_links, $text);

  # Return our modified text.
  return $text;
}

?>