Looks like this function has exactly the same behavior as base64_decode($test, true) -> it returns FALSE if $text contains characters outside the Base64 alphabet, although this isn't documented here.
(PHP 4, PHP 5, PHP 7, PHP 8)
imap_base64 — BASE64 でエンコードされたテキストをデコードする
stringエンコードされたテキスト。
デコードしたメッセージを文字列で返します。
失敗した場合に false を返します
Looks like this function has exactly the same behavior as base64_decode($test, true) -> it returns FALSE if $text contains characters outside the Base64 alphabet, although this isn't documented here.