Get Salesforce PickList values as CSV String

GetPickListValues.cls

Schema.DescribeFieldResult dfr = Opportunity.StageName.getDescribe();

List<Schema.PicklistEntry> picklistValues = dfr.getPicklistValues();

String output = '';

for(Schema.PicklistEntry entry : picklistValues) {
    if (output != '') {
        output += ',';
    }
    output += entry.getLabel();
}

System.debug(output);