There are many Symfony versions around, and it's good to know the differences between them to learn how the framework was evolving during these years. The first stable Symfony version — 1.0 — was released in the beginning of 2007 and was supported for three years. In mid-2008, version 1.1 was presented, which wasn't compatible with the previous release, and it was difficult to upgrade any old project to this.

Symfony 1.2 version was released shortly after this, at the end of 2008. Migrating between these versions was much easier, and there were no dramatic changes in the structure. The final versions of Symfony 1's legacy family was released nearly one year later. Simultaneously, there were two version releases, 1.3 and 1.4. Both were identical, but Symfony 1.4 did not have deprecated features, and it was recommended to start new projects with it. Version 1.4 had 3 years of support.

If you look into the code, version 1.x was very different from version 2. The company that was behind Symfony (the French company, SensioLabs) made a bold move and decided to rewrite the whole framework from scratch.

The first release of Symfony2 wasn't perfect, but it was very promising. It relied on Git submodules (the composer did not exist back then). The 2.1 and 2.2 versions were closer to the one we use now, although it required a lot of effort to migrate to the upper level. Finally, the Symfony 2.3 was released — the first long-term support version within the 2.x branch. After this version, the changes provided within the next major versions (2.4, 2.5, and 2.6) are not so drastic and usually they do not break compatibility.