Lab - 404 PHP handler

Implement the 404 PHP handler

• Get .htaccess working
• Write 404.php

Simple version

1. Find the bad URL:
   • 404.php extracts the bad URL using REDIRECT_URL in $_SERVER

2. Find the RHS part of the bad URL:
   • Split the bad URL into segments using explode on "/". This makes an array of segments.
   • Simple version: Assume the bad URL was "/~user/file" and just extract the RHS "file" bit.

3. Make a list of good URLs:
   • Make a pre-built list of all good URLs. Put this list in the file filelist.

4. Case-insensitive and partial search:
   • Search the segment in the list of good URLs, using a command-line call to something like:

      grep -i segment filelist 

   • This outputs the lines that match.

5. Make the hit linkable:
   • Simple version: Assume only one hit.
   • Collect the grep output using shell_exec
   • Output something like:

       
     print "<a href=$hit>$hit</a>"; 
     

More complex version

1. Sanitise all user supplied data, including user supplied URL.
   • Make the URL safe for grep by changing it to alphanumeric plus / plus "." for any other character. We use "." because it means "any character" in grep.
   • Try this using preg_replace:

      preg_replace ( "|[^A-Za-z0-9/]|", '.', $string ) 

2. Allow more complex bad URL: "/~user/dir/dir/file"
   • "explode" gives us an array of all the segments
   • We look at the RHS segments first. So reverse the array. There is a PHP function to do this.
   • Then go through the array with foreach.
   • Search each segment. Collect the return code from grep.
   • Exit when we got a match.

3. Assume each grep could have multiple matches
   • Split the return into an array
   • Print possibly multiple links