I used to write CAD system software (many moons ago), and when doing so I used what I call "defensive coding techniques". Which meant that I never trusted user input, that a file would be available, that I could write correctly to a device - I tested, tested and tested again. In our environment, it made huge sense. The application had to be installed on a wide variety of machines and be self configuring, and to report back in layman's terms about any problems. User inputs - typically made by draughtsmen who were excellent in their filed but very new to computers - had to be tested for the right type of content, and sensibility too. And checks had to be made on system resources to make sure that they really were available. It was no good just pouring the output to a pen plotter onto an RS232 port on which the plotter was switched off or off-line or out of paper - a meaningful trap and error message was needed.
So why is it that I see so many PHP web applications written in a non-defensive manner, and indeed I write non-defensive PHP pages myself?
* Web applications in PHP are running for hundreds or thousands of users on a single server, so the issues of installation and resources on lots of individual machines do not apply
* There are far few inputs to a PHP page, and it's running as a "batchlet", so that the inputs are far more controlled and easily validated
* There is no need to defend agaist array overruns due to dynamic memory allocation; the power of the computer is used to eliminate from within the language things that the authors of code have had to do for themselves for years.
* Modern language have far better error trapping and handling built in to them. So that although error conditions need to be checked, it's done at a very much higher level
* With PHP code being writeable so much faster than traditional code, you're working with a much smaller developer team - perhaps just one person where there would have been 3 had you been using C or Fortran. So there's no need for you to have as much testing and checking at the interface between various parts of your system - they're an individual's call to another of his functions and not a possible API misunderstanding between to members of a team
* With huge numbers of users on a single instance of the code, any error will be reported and shake out quickly; if the absence of defensive code leads to a problem it will be seen by a higher number but a lower proportion of users - soreported faster, but less upsetting to the user community.
You may not agree with all of the above, but look at each carefully and it's a justification for not writing code as defensively in PHP as you would have in C++.
My personal view?
There is no need to code as defensively as you would have done in the past os projects that (in PHP) are quite small - BUT - you should remember that when you develop what you think will be a small application, it's likely to end up turning into something bigger. And that you might want to share code later in which case a few extra checks and balances may be no bad thing.
And you should ALWAYS secure your PHP application against injection attacks!
OK - Let's answer that "the other way around" ;-)
Put another way, you should take a step back and when you right code you should decide WHAT you're defending against. And in a web / PHP environment it's different to what it was in a traditional coding environment!
You MAY be able to ease up on some of the traditional "defences" but you need to add things like ...
• Injection Attack considerations - technical (<, ", ' \ and space characters, file names that start / or ../, etc)
• Injection of undesired content (malicious and silly sign ups, content into anything that manages content such as blogs, forums)
• Action to ensure that the HTML you generate is high quality / always to standard - helps avoid browser dependency issues
• Issues with login systems / cookies / account hijacking
• User documentation issues - your users on the web are unlikely to call you if they can't understand how to use the site, where such issues would have been picked up during training on the CAD system
• Tests to ensure that robots (some of which are malicious) can't do large scale damage.
• protect code against installation on systems which may have register_globals on, or magic_quotes off, or short_tags off - which can make a difference to the source code.
• allow for system changes later - PHP4 to PHP5 if you copy objects, and modules such as mysql -> mysqli (or perhaps sqlite in the future?) (written 2008-07-02)
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