To use the IMAP functions in PHP you'll need to install the IMAP extension:

Debian/Ubuntu with PHP5

sudo apt-get install php5-imap
sudo php5enmod imap

Debian/Ubuntu with PHP7

sudo apt-get install php7.0-imap

YUM based distro

sudo yum install php-imap

Mac OS X with php5.6

brew reinstall php56 --with-imap

To do anything with an IMAP account you need to connect to it first. To do this you need to specify some required parameters:

• The server name or IP address of the mail server
• The port you wish to connect on
  • IMAP is 143 or 993 (secure)
  • POP is 110 or 995 (secure)
  • SMTP is 25 or 465 (secure)
  • NNTP is 119 or 563 (secure)
• Connection flags (see below)

Flag	Description	Options	Default
/service=service	Which service to use	imap, pop3, nntp, smtp	imap
/user=user	remote user name for login on the server
/authuser=user	remote authentication user; if specified this is the user name whose password is used (e.g. administrator)
/anonymous	remote access as anonymous user
/debug	record protocol telemetry in application's debug log		disabled
/secure	do not transmit a plaintext password over the network
/norsh	do not use rsh or ssh to establish a preauthenticated IMAP session
/ssl	use the Secure Socket Layer to encrypt the session
/validate-cert	certificates from TLS/SSL server		enabled
/novalidate-cert	do not validate certificates from TLS/SSL server, needed if server uses self-signed certificates. USE WITH CAUTION		disabled
/tls	force use of start-TLS to encrypt the session, and reject connection to servers that do not support it
/notls	do not do start-TLS to encrypt the session, even with servers that support it
/readonly	request read-only mailbox open (IMAP only; ignored on NNTP, and an error with SMTP and POP3)

Your connection string will look something like this:

{imap.example.com:993/imap/tls/secure}

Please note that if any of the characters in your connection string is non-ASCII it must be encoded with utf7_encode($string).

To connect to the mailbox, we use the imap_open command which returns a resource value pointing to a stream:

<?php
$mailbox = imap_open("{imap.example.com:993/imap/tls/secure}", "username", "password");
if ($mailbox === false) {
    echo "Failed to connect to server";
}

Once you've connected to your mailbox, you'll want to take a look inside. The first useful command is imap_list. The first parameter is the resource you acquired from imap_open, the second is your mailbox string and the third is a fuzzy search string (* is used to match any pattern).

$folders = imap_list($mailbox, "{imap.example.com:993/imap/tls/secure}", "*");
if ($folders === false) {
    echo "Failed to list folders in mailbox";
} else {
    print_r($folders);
}

The output should look similar to this

Array
(
    [0] => {imap.example.com:993/imap/tls/secure}INBOX
    [1] => {imap.example.com:993/imap/tls/secure}INBOX.Sent
    [2] => {imap.example.com:993/imap/tls/secure}INBOX.Drafts
    [3] => {imap.example.com:993/imap/tls/secure}INBOX.Junk
    [4] => {imap.example.com:993/imap/tls/secure}INBOX.Trash
)

You can use the third parameter to filter these results like this:

$folders = imap_list($mailbox, "{imap.example.com:993/imap/tls/secure}", "*.Sent");

And now the result only contains entries with .Sent in the name:

Array
(
    [0] => {imap.example.com:993/imap/tls/secure}INBOX.Sent
)

Note: Using * as a fuzzy search will return all matches recursively. If you use % it will return only matches in the current folder specified.

You can return a list of all the messages in a mailbox using imap_headers.

<?php
$headers = imap_headers($mailbox);

The result is an array of strings with the following pattern:

[FLAG] [MESSAGE-ID])[DD-MM-YYY] [FROM ADDRESS] [SUBJECT TRUNCATED TO 25 CHAR] ([SIZE] chars)

Here's a sample of what each line could look like:

A     1)19-Aug-2016 [email protected] Message Subject (1728 chars)
D     2)19-Aug-2016 [email protected] RE: Message Subject (22840 chars)
U     3)19-Aug-2016 [email protected] RE: RE: Message Subject (1876 chars)
N     4)19-Aug-2016 [email protected] RE: RE: RE: Message Subje (1741 chars)

Symbol	Flag	Meaning
A	Answered	Message has been replied to
D	Deleted	Message is deleted (but not removed)
F	Flagged	Message is flagged/stared for attention
N	New	Message is new and has not been seen
R	Recent	Message is new and has been seen
U	Unread	Message has not been read
X	Draft	Message is a draft

Note that this call could take a fair amount of time to run and may return a very large list.

An alternative is to load individual messages as you need them. Your emails are each assigned an ID from 1 (the oldest) to the value of imap_num_msg($mailbox).

There are a number of functions to access an email directly, but the simplest way is to use imap_header which returns structured header information:

<?php
$header = imap_headerinfo($mailbox , 1);

stdClass Object
(
    [date] => Wed, 19 Oct 2011 17:34:52 +0000
    [subject] => Message Subject
    [message_id] => <04b80ceedac8e74$51a8d50dd$0206600a@user1687763490>
    [references] => <[email protected]>
    [toaddress] => Some One Else <[email protected]>
    [to] => Array
        (
            [0] => stdClass Object
                (
                    [personal] => Some One Else
                    [mailbox] => someonelse
                    [host] => example.com
                )
        )
    [fromaddress] => Some One <[email protected]>
    [from] => Array
        (
            [0] => stdClass Object
                (
                    [personal] => Some One
                    [mailbox] => someone
                    [host] => example.com
                )
        )
    [reply_toaddress] => Some One <[email protected]>
    [reply_to] => Array
        (
            [0] => stdClass Object
                (
                    [personal] => Some One
                    [mailbox] => someone
                    [host] => example.com
                )
        )
    [senderaddress] => Some One <[email protected]>
    [sender] => Array
        (
            [0] => stdClass Object
                (
                    [personal] => Some One
                    [mailbox] => someone
                    [host] => example.com
                )
        )
    [Recent] =>  
    [Unseen] =>  
    [Flagged] =>  
    [Answered] =>  
    [Deleted] =>  
    [Draft] =>  
    [Msgno] =>    1
    [MailDate] => 19-Oct-2011 17:34:48 +0000
    [Size] => 1728
    [udate] => 1319038488
)