PHP has a native password_hash function as of version 5.5, so trying to define a function with the same name (without using a namespace) will result in that error you’re getting.
Probably the simplest solution is to rename the function in your code to something that doesn’t conflict with any native functions. Obviously you’ll have to make sure you rename all uses of the function within the code (do a seach and replace). The downside of this is that if you upgrade the script to a new version in the future then your changes will be overwritten and your app will break again.