Looks like this function has exactly the same behavior as base64_decode($test, true) -> it returns FALSE if $text contains characters outside the Base64 alphabet, although this isn't documented here.
(PHP 4, PHP 5, PHP 7, PHP 8)
imap_base64 — Decodifica texto codificado em BASE64
Decodifica a string codificada em BASE-64 fornecida.
stringO texto codificado
Retorna a mensagem decodificada como uma string, ou false em caso de falha.
Looks like this function has exactly the same behavior as base64_decode($test, true) -> it returns FALSE if $text contains characters outside the Base64 alphabet, although this isn't documented here.