Можно попробовать обойтись без массивов.
На примере gmail.
$imapPath = '{imap.gmail.com:993/imap/ssl/novalidate-cert}INBOX';
$username = 'login@gmail.com';
$password = 'password';
$inbox = imap_open($imapPath,$username,$password) or die('Cannot connect to Gmail: ' . imap_last_error());
// Тут главное действие, поиск в папке "Входящие"
$emails = imap_search($inbox,'TEXT "yii"'); // найти письма с текстом "yii"
// или
$emails = imap_search($inbox,'FROM "exsample@mail.com"'); // письма с определённым адресом
Вот полный список
ALL - return all messages matching the rest of the criteria
ANSWERED - match messages with the \\ANSWERED flag set
BCC "string" - match messages with "string" in the Bcc: field
BEFORE "date" - match messages with Date: before "date"
BODY "string" - match messages with "string" in the body of the message
CC "string" - match messages with "string" in the Cc: field
DELETED - match deleted messages
FLAGGED - match messages with the \\FLAGGED (sometimes referred to as Important or Urgent) flag set
FROM "string" - match messages with "string" in the From: field
KEYWORD "string" - match messages with "string" as a keyword
NEW - match new messages
OLD - match old messages
ON "date" - match messages with Date: matching "date"
RECENT - match messages with the \\RECENT flag set
SEEN - match messages that have been read (the \\SEEN flag is set)
SINCE "date" - match messages with Date: after "date"
SUBJECT "string" - match messages with "string" in the Subject:
TEXT "string" - match messages with text "string"
TO "string" - match messages with "string" in the To:
UNANSWERED - match messages that have not been answered
UNDELETED - match messages that are not deleted
UNFLAGGED - match messages that are not flagged
UNKEYWORD "string" - match messages that do not have the keyword "string"
UNSEEN - match messages which have not been read yet
p.s.
Но есть одно "НО". Не знаю как с другими почтовиками, но в gmail необходимо разрешить доступ для непроверенных приложений и подтвердить свой выбор через письмо, которое присылается на Ваш ящик автоматически.