Don't reinvent the wheel — unless the wheel is written in PHP, opinionated in ways you'll always be fighting against and not a wheel, but a steaming pile of hot code. This site is not a Wordpress site and I am so happy I "reinvented the wheel." I chose a stack — Rails — that I enjoy working with and gave me just the right balance between constraint and freedom for the project. Rails is not "better" than Wordpress. They are apples and oranges, but laying the concrete myself has given me the ability to truly make the thing I want to make. Also, in my day job I don't get the simple pleasures of building a basic CRUD app, don't take that away from me naysayers!
A lot of the time, yes, you should reach for the existing implementation, but before you ever do that, you should go through the exercise of thinking about the consequences and reaching some sort of justification one way or the other, even if you have incomplete information, and you will have incomplete information.
Rarely will you be able to come up with the complete answer, but sometimes it is about the journey and not the destination. Thinking through the consequences of whether to write it yourself or use a library/framework/service is a very useful exercise in helping see into the future of your technical needs.
Writing software is a never-ending sequence of jumping and hoping you land on something soft. You never know what future needs will be but avoiding blanket rules like "not reinventing the wheel." Don't be afraid to "waste time" by taking a walk to think about everything that will go into the decision and what the future might hold. Are you saving time now but adding burden later? It will pay off.