PHP Mysqli select to csv Options

[nuddernuby]

Could someone save me from insanity and provide a working script for the following simple problem, please. (I've tried many and failed)

I have a table 'projects' with field 'projectname', currently populated with five records, say A,B,C,D & E.
I need to store an the array of these records in this format: $arr=("A","B","C","D","E") or output it to a csv string in csv file.

I can echo the results but fail to get this into the required format/file.

Basic query is:
$query=mysqli_query($conn,"SELECT projectname FROM projects");

What then?

Apologies for the simplicity of the problem.