Does Writing Files First Erase?

Question

Part of the course says that:
w: Opens file for write only. Erases the contents of the file or creates a new file if it doesn't exist.

Does this mean that if you want to write something onto a file, you should use "a" if you want to keep what is already there?
Or do you have to first read a file and then write to it what you read plus what you wanted to add?

Answer 1

Yes, using the flag 'a' allows you to append to the file instead of overwriting it without having to read in the content first:

https://www.sololearn.com/learn/PHP/1846/