Studying the evolution of PHP web applications

Abstract

Context: Software evolution analysis can reveal important information concerning maintenance practices. Most of the studies which analyze software evolution focus on desktop applications written in compiled languages, such as Java and C. However, a vast amount of the web content today is powered by web applications written in PHP and thus the evolution of software systems written in such a scripting language deserves a distinct analysis. Objective: The aim of this study is to analyze the evolution of open-source PHP projects in an attempt to investigate whether Lehman’s laws of software evolution are confirmed in practice for web applications. Method: Data (changes and metrics) have been collected for successive versions of 30 PHP projects while statistical tests (primarily trend tests) have been employed to evaluate the validity of each law on the examined web applications. Results: We found that Laws: I (Continuing Change), III (Self regulation), IV (Conservation of organizational stability), V (Conservation of familiarity) and VI (Continuing growth) are confirmed. However, only for laws I and VI the results are statistically significant. On the other hand, according to our results laws II (Increasing complexity), and VIII (Feedback system) do not hold in practice. Finally, for the law that claims that quality declines over time (Law VII) the results are inconclusive. Conclusions: The examined web applications indeed exhibit the property of constant growth as predicted by Lehman’s laws and projects are under continuous maintenance. However, we have not found evidence that quality deteriorates over time, a finding which, if confirmed by other studies, could trigger further research into the reasons for which PHP web applications do not suffer from software ageing.