I was going to post about a little problem that I experienced while undertaking a project I’m involved with but canned it thinking it was of limited interest. That was, however, until I made the same mistake again yesterday, so I thought I would document it to save my own sanity, even if there is no-one else who cares – not that anyone reads this anyway.

So to my faux pas, and believe it or not this cost me a few hours! The problem: well as part of my project (a bike shop website for those who really want to know – www.cyclewize.com) I’m programming in PHP using cakePHP. The advantages/disadvantages of PHP can be debated from dusk till dawn if that is your thing – just remember though, while you are debating all those smart people are just getting on with doing stuff. That said, my problem was, in a sense, due to the use of PHP or dynamic languages in particular.

Anyway, back on track, so the component I was working on simply uploads a set of photos and scales them accordingly. Simple as that. The problem that I kept seeing was that although the photographs were being uploaded ok, i.e no errors were being reported, I was unable to copy the files from their temporary directory over to their new home. Since no errors were being returned I was stuck as to what was happening. I battled away putting print_r’s in here there and everywhere to no avail. Then I just happened to check my call to move_uploaded_file. On inspection, I noticed I had a slight misspelling of a variable name in one of the functions arguments, thus only at that point did the variable come into existence. Therefore what was getting passed to the function was the empty string and hence move_uploaded_file could not copy the file – you would have thought it should have raised an error but no. However, as it turns out, even if it had raised an error it wouldn’t have helped me much. Why? Because straight after the call to move_uploaded_file I was doing a redirect to a new page. Therefore, any errors related to the problem http request were output to a screen which flashed by too quickly for me to notice.

So what is the root of this problem? Well if I was using a complied language it is likely that the compiler would have warned me that the variable I was passing to move_uploaded_file was either not declared or at least never been initialised. I’m pretty sure that a warning message would have been output (which I missed) by PHP’s interpreter but this is no substitute for a compile-time message. As much as I love the freedom of dynamic languages you have to be aware of this significant drawback, and people can go on about how this stuff gets picked up through thorough testing but I’m not sure that’s good enough. Also, the page redirect was a killer, as it made me miss all the output messages. It’s probably worth while not performing redirects if you find something isn’t working, just to ensure that you are seeing all the messages output by the interpreter (you can always switch it back on later). Finally, some sort of lint check should have picked this up. So from now on I think I will be running things through php –l to check that my syntax is good.

There you have it, a nice root cause analysis! I probably missed out the fact that it was my rather inane programming that was the main problem, but bugger it, I’m not really into damaging my ego.