Looks like this function has exactly the same behavior as base64_decode($test, true) -> it returns FALSE if $text contains characters outside the Base64 alphabet, although this isn't documented here.
(PHP 4, PHP 5, PHP 7, PHP 8)
imap_base64 — Décode un texte encodé en BASE64
Décode le texte string encodé en BASE64.
stringLe texte encodé.
Retourne le texte décodé, sous la forme d'une chaîne de caractères, ou false si une erreur survient.
Looks like this function has exactly the same behavior as base64_decode($test, true) -> it returns FALSE if $text contains characters outside the Base64 alphabet, although this isn't documented here.