Clearly you do not know how secure it is as said here https://dev.acquia.com/podcast/69-php-secure-it-if-you-do-it-right-anthony-ferrara. It is secure if you do it right. If you do not understand PHP you should most likely not use it for production use because of your lack of knowledge. Also...
If you want to code a nice, smooth, and fast iOS / Android app you probably want to use Swift, Java, JS, PHP, and Objective C. The main reason you want to use Swift is because it is now open source and with that being said i can see in the future that Swift will soon be compatible on both platforms.
Honestly i would have to go with Slim Framework all the way their router is by far the fastest i have seen and how easy it is to use makes it great. With composer makes it even better because with one command line it is already installed. Slim Framework is always updated and the documentation...
PHP is at the top of my list due to the fact you can do so much with it, For example you can use it to send emails, send phone sms messages, custom URL/json/XML encode functions so if you want to echo a response in json just do json_encode(Your Response). It is also the best security wise. PHP...