Apex domains cannot have CNAME records.
Question and response
Question
dig php-friends.de. NS
Answer
php-friends.de. 5 NS ns2.dataforest.net.
php-friends.de. 5 NS ns1.dataforest.net.
php-friends.de. 5 NS ns4.dataforest.net.
php-friends.de. 5 NS ns3.dataforest.net.
Authority
The response has no authority records.
Additional
ns1.dataforest.net. 5 A 62.116.159.4
ns2.dataforest.net. 5 A 217.160.113.37
ns3.dataforest.net. 5 A 89.146.248.4
ns4.dataforest.net. 5 A 74.208.254.4
ns1.dataforest.net. 5 AAAA 2001:4178:3:a357:62:116:159:4
ns2.dataforest.net. 5 AAAA 2001:8d8:580:401:217:160:113:37
ns3.dataforest.net. 5 AAAA 2a01:130:2000:118:89:146:248:4
ns4.dataforest.net. 5 AAAA 2607:f1c0:1800:8091:74:208:254:4
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0