PHP is a general-purpose server-side scripting language originally designed for Web development to produce dynamic Web pages, whereas Ruby on Rails, is an open source full-stack web application framework for the Ruby programming language. So the primary difference to keep in mind is that PHP is a LANGUAGE and Ruby on Rails is a FRAMEWORK. In a border sense, PHP & Ruby cannot be compared.
PHP is simply a programming language. However, it is an extremely flexible and useful language. PHP uses very little memory and performs quite well. It is also a mature language with a wide variety of extensions and library modules. Originally developed as a scripting language to be used in HTML pages, PHP has gradually matured and now includes full object oriented features. However, some programmers feel that PHP encourages sloppy coding because of its script-like roots and lack of a clear object oriented strategy from the beginning.
In contrast, Ruby on Rails is a framework developed in the Ruby programming language. Ruby was designed from the ground up with object oriented design in mind. It has a very tight and clear syntax and programming in it can be down simply. Though, sometimes it can also be extremely inflexible. As a programming language, Ruby often runs more slowly than PHP and requires more memory. Rails is a framework that provides a number of features that are useful for the rapid development of web applications.
So at the end of the day, it all depends on the platform that you already know, that will do what you need and that your clients need.