Looks like this function has exactly the same behavior as base64_decode($test, true) -> it returns FALSE if $text contains characters outside the Base64 alphabet, although this isn't documented here.
(PHP 4, PHP 5, PHP 7, PHP 8)
imap_base64 — Dekodiert Base64-kodierten Text
Dekodiert den angegebenen Base64-kodierten string.
stringDer zu dekodierende Text
Gibt den dekodierten Text als String zurück. Bei einem Fehler wird false zurückgegeben.
Looks like this function has exactly the same behavior as base64_decode($test, true) -> it returns FALSE if $text contains characters outside the Base64 alphabet, although this isn't documented here.