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 — Decodificar un texto cifrado con BASE64
$text): string
Decodifica el text cifrado en BASE-64 dado.
textEl texto cifrado
Devuelve el mensaje decodificado como una cadena.
Looks like this function has exactly the same behavior as base64_decode($test, true) -> it returns FALSE if $text contains characters outside the Base64 alphabet, although this isn't documented here.