There are a number of IDE's out there which are designed for or can be configured for PHP. Which one is best for you will depend on your development operating system, your target operating system, your PHP version, and your personal taste.
search for "free PHP IDE" and see what takes your fancy.
I generally code by hand in vim, using libraries to minimise wheel redevelopment. But that's not for everyone. If I'm badly hungover or otherwise perfroming suboptimally, I might just use a graphical editor like kate.
notepad? shudder