Why do base64/openssl use a padding character of 'K' instead of '='

Ive noticed that php base64_encode uses '=' as a padding character. According to Wikipedia the different types use either '=' or none. However the CLI base64 command as well as openssl enc -base64 use 'K' as the padding. I am looking for information as to why and what implementations they use.

echo base64_encode('hello'); // aGVsbG8=
echo hello | base64 -i - // aGVsbG8K
openssl enc -base64 <<< hello   // aGVsbG8K