PHP flatfile storage
Assuming this code is solely for learning purpose, there are several main issues


to be honest, a function that returns ERR_DX_FILE_DOES_NOT_EXIST when I request an object looks a bit weird. I don't even know how to use it. Should I wrap every call to a function in a condition that checks the returned value? Or what else I am supposed to do with this string? Consider using errors/exceptions in case of error.
a race condition issue. you can have this file malformed if two parallel processes would try to write at the same time. Consider use file locks
an obvious memory issue - the bigger the file is, the more RAM does it take to process. PHP can let you to read a file line by line without memory overhead
such a clumsy format as keyname__=__urlencode(serialize(value_of_key)) doesn't seem to be very reliable. For such a toy storage I would rather use json format, to ease the search/key access.