In the early two-thousands, corporate applications were built with C, Java, or sometimes ASP. At that time, PHP wasn't welcome in the enterprise world yet. But despite the lack of clearance from the CIOs, PHP was already inside the corporate walls.
It all started with this IT guy. You know who I'm talking about: this kinda hippie guy, who was unaware of the corporate rules, or not willing to obey them. He installed a simple blog engine, or a photo album engine, or a bulletin board engine, on a disused server. These applications had no interface with the corporate IT systems. PHP arrived with simple tools that were much easier to deal with than the state-of-the-art technology. And the IT guy started to learn the language, and understood that it was able to do much of the stuff the CIOs asked to Java or ASP. And that's how PHP sneaked into the enterprise. Today, PHP is broadly accepted as an industry-class language, and adopted by large banks, medias, retailers, for both frontend and backend applications. In the meantime, PHP had to become a robust language with industrial tools (frameworks, automatic build, monitoring, etc). And lost part of its attraction in the process.