Recently (well in a loose sense anyway) I had the need to build a document bank in PHP for a client at Mosaic. It was a fairly involved application with various public and private APIs for integration into the clients network of websites.

The core PHP code was written on top of the Agavi framework and various PHP libraries for extracting text and meta data from documents. One of the major features the client required was for the system to detect similar files to prevent unintentional duplicates making it into the document bank.

The idea was that this document bank would be the one central resource for all of the documents written and managed by the organisation. Duplicates or near duplicates would of course make this a pointless exercise. So I turned to StackOverflow for some pointers, but came up empty.

After some research and much searching of the web I came across an open source package called ssdeep written by Jesse Kornblum. I found it through reading his research papers; Identifying almost identical files using context triggered piecewise hashing.

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Installing via the pecl command can be a pain on Redhat. First off all you will need to install the php-devel package:

yum install php-devel

Then you will need ensure that the PEAR/PECL installer is at the latest version so as root run:

pear update-channel pear.php.net
pear upgrade pear

You may need to force pear to upgrade itself by using:

pear upgrade —force pear

I had to use the —force option because my version of PEAR was so old that the installer thought my version of Tar_Archive might not have been up to muster. It was however.

With all this in place you are ready to attempt to install your chosen extension:

pecl install ssdeep

If you get something like the following back:

/usr/bin/phpize: /tmp/ssdeep/build/shtool: /bin/sh: bad interpreter: Permission

Then it is likely that your temporary directory is mounted in a safer noexec state, which means that you cannot execute scripts within the /tmp directory. To test this you can put a simple bash script into your /tmp directory and chmod it with +x. I used the following bash script:

#!/bin/bash
echo “SIMON” 

If you do not get SIMON back when you execute the file, but an error like “/bin/sh: bad interpreter: Permission” then the directory is set to noexec.

There are a few ways to overcome this with the easiest being to:

1. Remount the directory as exec:

   mount -o remount,exec /tmp

2. Install your pecl extension

   pecl install ssdeep

3. Remount the directory as noexec again for safety

   mount -o remount,noexec /tmp

There are a variety of other ways to get this working documented in the Media Temple wiki pages if the above technique does not work for you.

Whilst developing a PHP extension recently I spent quite a bit of time researching exactly how to create an extension, the best practices and the DocBook format of the PHP manual for documenting the extension.

By the time I finished writing the extension I had found some very good resources both on the web and in print.

Online Articles or Websites:

• The Internals section of the PHP manual
• Wez Furlong’s Extending PHP slides
• Marcus Börger and Johannes Schlüter: PHP Extension Writing slides
• O’Reilly Programming PHP Chapter 14: Extending PHP
• Sara Golemon’s Zend DevZone articles with parts I, II and III:
  • Extension Writing Part I: Introduction to PHP and Zend
  • Extension Writing Part II: Parameters, Arrays, and ZVALs
  • Extension Writing Part III: Resources
• Paul Osman’s Wrapping C++ Classes in a PHP Extension article on Zend DevZone
• Kristina Chodorow’s PHP Extensions Made Eldrich on her blog

Printed Books:

• Sara Golemon’s Extending and Embedding PHP
• Advanced PHP Programming by George Schlossnagle

Information on Documenting PHP and PECL Extensions:

• PHP Wiki: Documentation Introduction
• PHP Wiki: Editing Documentation
• PHP Documentation Team: How to pages
• PHP Wiki: Documenting PECL Extensions

Also worth a mention is the ability of the new PEAR2 Pyrus installer to generate a skeleton for you to build your PECL extension upon - see the generate-ext section of the manual.

Pyrus can also be used to package your extension up for release using the pickle command.

For more help there is a PECL mailing list and IRC channel (#php.pecl on efnet), both of which are in the support section of the PECL website.

Whilst not strictly for PHP extensions the Autotools: a practitioner’s guide to Autoconf, Automake and Libtool book by John Calcote on the Free Software Magazine is very useful reading for an understanding of the build process involved in extension writing.

There is also the GNU Manual for Autoconf and GNU Autoconf, Automake, and Libtool by Gary V. Vaughan et al.