1. Maintenance Patterns of large-scale
PHP Web Applications
Panos Kyriakakis1 and Alexander Chatzigeorgiou2
1 Hellenic Open University
2 University of Macedonia, Greece
ICSME’2014, Research Track, October 3, 2014

2. Ewww, You Use PHP?
As we said in our teaser video, there are various sources in the Internet, mainly anecdotal, that claim that
Scripting languages are not appropriate for proper software engineering
Not appropriate for building large projects that should be maintained
Not appropriate for teaching CS1
but you can hardly find any research evidence on which these claims are based
Most of the features offered by scripting languages are not offered by Java

3. Stanford : Javascript (variant) Berkeley: Python
Systematic argumentation on the benefits of scripting languages has been developed
CS101 in:
MIT : Python
Stanford : Javascript (variant)
Berkeley: Python
Harvard: C, PHP, Javascript
Cornell: Python

4. Five large open-source PHP Web Applications Goal of this work
Study the evolution of
Five
large
open-source
PHP
Web Applications
In terms of quality, maturity and migration to OO

5. Case Study Design 2,674 themes, 110 million downloads
492 contributors for Drupal 5.0
most widely used open source bulletin board system
PhpBB is by far the most widely used open source bulletin board system
Started in 2000, 1.8 M downloads in sourceforge, translated in 50 languages, ~ 2577 downloads / week
“born” in > 200,000 downloads / month
669 contributors, estimated effort: 136 person-years

6. Analyzed Projects Project Years Releases First Last Release KLOC
Code Blocks
WordPress 9 71
1.5 20
763
3.6.1
200
5154
Drupal 12
120
4.0.0 14
692
7.23
173
5140
phpBB 37
2.0.0 18
256
3.0.12
194
2389
MantisBt 8 33
1.0.0 68
2447
1.2.15
165
4904
phpMyAdmin
129
2.9.0 39
693
4.1.6
248
5343
Some of the projects have their roots back before PHP fully supported object-orientation, providing the opportunity to investigate the migration.
390 official releases
Lehman's 6th law confirmed
50 years of evolution
>36 million LOC

7. The Approach
Downloading
Bergmann’s PHP DCD
AST analysis

8. Survival Analysis
Survival analysis models the time it takes for events to occur
why do we need survival analysis?
Start of the study
End of the study
censored
???????
censored
Survival analysis models the time it takes for events to occur and focuses on the distribution of the survival times
Let us assume that we observe patients over time and monitor the time it takes for an event to occur (unfortunately, this is usually death)
???????
censored
time

9. Kaplan-Meier curves C(ti)=S(ti)*C(ti-1)
Survival function S(ti)=1-di/ri
Cumulative Probability of surviving up to time t
C(ti)=S(ti)*C(ti-1)
If this was a real treatment then you would conclude that treatment for Group1 is more successful

10. Survival Analysis: function usage in PHP
Termination event: a function becomes unused

11. function usage in PHP (2)
Survival Analysis  Way of identifying hotspots
Stalactites= hotspots or POI

12. function usage in PHP (3)
Survival function S(ti)=1-di/ri
method Additions (%)
nuSoap library
ADOdb library
For three of the projects a common pattern can be observed. Drops in the survival function appear when employment of libraries takes place.
In scripting languages like PHP, using a third party library, implemented also in PHP, means that the library's code has to be added to the application's code. Unavoidably, using a proportion of the library's functionality leads the rest of the library code to remain unused.
However there is also another reason for which we see methods that are not being used and that is the hooking mechanism

13. Hooks Stealth calls /* global array of registered hooks */
$hooks = array();
/* function for registering hooks */
function add_action($hook, $funcName) {
global $hooks;
if( !isset($hooks[$hook]) ) {
$hooks[$hook] = array(); }
$hooks[$hook][] = $funcName;
/* function for executing a particular hook */
function do_action($hook) {
foreach($hooks[$hook] as $fn) {
call_user_func($fn);
/* extensible function */
function myFunc() {
/* ... function code ... */
do_action('my_hook');
/* extension */
function myHookFunc() { }
Stealth calls
A simplified example on the widely used practice of hook functions. Hooks offer a functional implementation of the observer pattern
/* registering a hook function */
add_hook('my_hook','myHookFunc');

14. function usage in PHP (3)
Stalactive – Stalagmite phenomenon
In general drops in the survival function coincide with major versions.
A stalactite-stalagmite phenomenon is evident: new code is added in major versions, where the introduction of unused code is taking place as well (which can be either due to the introduction of libraries or the use of hooks, both of which imply major architectural changes)

15. Survival Analysis: function removal
Termination event: a function is removed from the system

16. 45% replaced with methods
function removal in PHP (2)
Survival function S(t)
Percentage of added methods
45% replaced with methods
62% of removed functions
replaced with methods
It is important to mention that whenever functions are replaced with methods, it is entire pieces of functionality that migrate to classes

17. function removal in PHP (3)
However, not all systems convert existing functions to methods
Functions are replaced with functions
Only new functionality written in OO
In contrast Drupal and Wordpress do not convert existing functions to methods. They only implement new functionality with OO. The reason is that in these systems there are thousands of plugins (more than 30,000 in Wordpress) and reimplementation of existing functionality with methods would lead to compatibility problems. So, the designers of these systems are extremely cautious
Why ?
Wordpress: > 30,000 plugins, > 2,000 themes
Drupal: > 8,000 plugins, >600 themes

18. Library usage
“pre-made building blocks ease and speed up the development of applications” (Tulach, Practical API Design)
“external libraries and their usage have a significant impact on the maintenance of the including software” (Bauer et al., ICSM’2012)
In PHP library code becomes part of the system’s source code  Easy to measure
There are various findings regarding the importance of libraries in software development like Tulach who concludes that…

19. Library usage (2) added PHPExcel
Some of the systems like phpBB and Drupal employ a limited portion of third party libraries
Drupal gradually replaced third party libraries with in-house code trying to build its own ecosystem and delivering all components under a single license
All other systems are gradually introducing library code at a steady pace.
PhpMyAdmin is quite interesting where for a time period library code was equal to system code.
So we see major architectural decisions such as the choice to rely on a particular library or not

20. Interface Stability Category Impact Severity Freq. C1 Change
Category
Impact
Severity C1
Change
of mandatory parameters
Breaking function's compatibility
Freq.
0.09%
0.15%
0.02%
0.94%
0.66% C2
Addition of optional parameters
No impact C3
Removal of optional parameters
Possible breaking of function's compatibility C4
Change of default values
Possible breaking of function's compatibility C5
Change of function's return type
Possible breaking of function's compatibility C6
Change of function's implementation
No impact
7.46%

21. Migration to OO
Almost OO
Turn to OOP
Various trends here,

22. Evolution of Complexity
[0,5)
Evolution of Complexity
[5,10)
percentage of functions with CCN in three ranges
>10

23. Overview of Findings × ✔ Main source of unused code is library usage.
Phenomenon
Conclusion
Survival regarding function usage 
Main source of unused code is library usage. × ✔
Unused code appears in major versions
Survival regarding function removal
Function removal appears in major versions
Only new features are implemented with OO
Functional code replaced with OO code
Library usage
Projects reuse code from third party libraries
Interface stability
Interface remains stable
Migration to OO
Projects migrate to OO
Evolution of Complexity
Complexity remains stable

24. Future Work
Embed in our platform SEAgle which currently analyzes Java projects
Compare the evolution of large and less known PHP projects

25. Conclusion Large PHP projects undergo systematic maintenance
PHP does not seem to hinder it
scripting languages can support the maintenance of evolving software projects and that maintainability is rather an issue of appropriate software engineering practices rather than an issue of the selected programming language.

26. Thank you for your attention!!
(and have a safe trip back home)
ICSME’2014, Research Track, October 3, 2014