Yes. If you are skilled and if you are interested in both languages then it would be great for your knowledge and it will also add so many career opportunities for you.
PHP is one of the most light weighted server side open source language and Asp.net is dynamic development based server side language. They both play important role in programming and development ad they both adds more functionality to the web pages. PHP is the development of The PHP Group and it was introduced in the year 1995.
Asp.net stands for Active Server Pages and it is developed by Microsoft. It was introduced in the year 2002 it is used for web application creation and it works with any .Net language. It is creative and there are so many amazing and easy to use functions in it. Additionally, it is said to be a secure web language because Microsoft has integrated advanced security functions in Asp.net language.
PHP is one of the most preferred languages for scripting and servers so being proficient in it would be definitely beneficial for you and if you will also consider about asp.net server side language then you would be able to produce dynamic functionality based web pages. You can add many exciting features to a simple web page with the help of some asp.net commands. This is best for making your web page multi purpose page and it would be great for web page creativity.
You can work as a junior php developer and it will help you to learn more about php and when you became proficient then you can consider learning other development languages such as asp.net. This will help you to be expert and this will make your creative skills deeper and broader. There are so many PHP and asp net developer jobs so that you can rest assured about your career.