Open source software is similar in idea to "free software" but slightly less rigid than the free software movement. Users of open source software are (generally) able to view the source code, alter and re-distribute open source software. There is however less of an emphasis in the open source movement on the right of information and source code to be free and in some cases companies are able to develop proprietary products based on open source ones.
The advantages of open source software are as follows:-
The availability of the source code and the right to modify it is very important. It enables the unlimited tuning and improvement of a software product. It also makes it possible to port the code to new hardware, to adapt it to changing conditions, and to reach a detailed understanding of how the system works. This is why many experts are reaching the conclusion that to really extend the lifetime of an application, it must be available in source form. In fact, no binary-only application more than 10 years old now survives in unmodified form, while several open source software systems from the 1980s are still in widespread use (although in many cases conveniently adapted to new environments). Source code availability also makes it much easier to isolate bugs, and (for a programmer) to fix them.
The right to redistribute modifications and improvements to the code, and to reuse other open source code, permits all the advantages due to the modifiability of the software to be shared by large communities. This is usually the point that differentiates open source software licences from ``nearly free'' ones. In substance, the fact that redistribution rights cannot be revoked, and that they are universal, is what attracts a substantial crowd of developers to work around open source software projects.
The right to use the software in any way. This, combined with redistribution rights, ensures (if the software is useful enough), a large population of users, which helps in turn to build up a market for support and customization of the software, which can only attract more and more developers to work in the project. This in turn helps to improve the quality of the product, and to improve its functionality. Which, once more, will cause more and more users to give the product a try, and probably to use it regularly.
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Here are the links to the most popular open source programs written in php:-