Your client, and/or the company you got hired in, uses PHP, Zend, WordPress, Drupal, Laravel, Symphony, CodeIgniter, Smarty, etc.
You need a proven server-side language for rapid development.
You can’t stand the Node.js and Ruby fanboys.
Your friend is using Python and you want to finish your project a week earlier.
Your other friend is on ASP.NET and you don’t mind not keeping the friendship.
You can’t stand offside-rule syntax and prefer C-like curly braced code.
You’re not pedantic about dynamic typing and inconsistent functions.
You love straightforward POST and GET requests.
You know mixing HTML and programming code is bad, but enjoy the fact that you can do it easily anyway if you wanted to.
You enjoy just saying that 80%+ of the web is written in your language of choice, especially major sites like Facebook, Wikipedia, and Google (even though they also use other languages, probably even more so).
You find the story of PHP starting out as a pet project templating language and eventually eclipsing other languages that took themselves far too seriously quite amusing and ironic.
You’re a contrarian and love using a language that everyone hates.
You don’t care too much about developer surveys and Quora questions like these written by fanboys (see #3) who love to hate on PHP.
You’re a two-faced sociopath and claim to hate PHP, but use it anyway and a lot behind closed doors.
You’re a cool developer, and know what PHP is good for and use it for those cases, but also know enough to use other languages when needed as well, like a really chill and cool developer.