I noticed system core files contain very good implementations such as single quotes for non-interpolated strings, strict return and parameter datatypes, strict equality and yoda conditions, good variable names and such. However, I also noticed almost half of the codes don't implement these.
These items may or may not be included during code reviews but it would be nice to have them implemented properly in the CI4 core in order to utilize PHP7's capabilities. If so, these conventions may need to be included in the User Guide's Contributing section as well.
Long live CodeIgniter!
