if someone need more information about curl errors
<?php
$error_codes=array(
[1] => 'CURLE_UNSUPPORTED_PROTOCOL', 
[2] => 'CURLE_FAILED_INIT', 
[3] => 'CURLE_URL_MALFORMAT', 
[4] => 'CURLE_URL_MALFORMAT_USER', 
[5] => 'CURLE_COULDNT_RESOLVE_PROXY', 
[6] => 'CURLE_COULDNT_RESOLVE_HOST', 
[7] => 'CURLE_COULDNT_CONNECT', 
[8] => 'CURLE_FTP_WEIRD_SERVER_REPLY',
[9] => 'CURLE_REMOTE_ACCESS_DENIED',
[11] => 'CURLE_FTP_WEIRD_PASS_REPLY',
[13] => 'CURLE_FTP_WEIRD_PASV_REPLY',
[14]=>'CURLE_FTP_WEIRD_227_FORMAT',
[15] => 'CURLE_FTP_CANT_GET_HOST',
[17] => 'CURLE_FTP_COULDNT_SET_TYPE',
[18] => 'CURLE_PARTIAL_FILE',
[19] => 'CURLE_FTP_COULDNT_RETR_FILE',
[21] => 'CURLE_QUOTE_ERROR',
[22] => 'CURLE_HTTP_RETURNED_ERROR',
[23] => 'CURLE_WRITE_ERROR',
[25] => 'CURLE_UPLOAD_FAILED',
[26] => 'CURLE_READ_ERROR',
[27] => 'CURLE_OUT_OF_MEMORY',
[28] => 'CURLE_OPERATION_TIMEDOUT',
[30] => 'CURLE_FTP_PORT_FAILED',
[31] => 'CURLE_FTP_COULDNT_USE_REST',
[33] => 'CURLE_RANGE_ERROR',
[34] => 'CURLE_HTTP_POST_ERROR',
[35] => 'CURLE_SSL_CONNECT_ERROR',
[36] => 'CURLE_BAD_DOWNLOAD_RESUME',
[37] => 'CURLE_FILE_COULDNT_READ_FILE',
[38] => 'CURLE_LDAP_CANNOT_BIND',
[39] => 'CURLE_LDAP_SEARCH_FAILED',
[41] => 'CURLE_FUNCTION_NOT_FOUND',
[42] => 'CURLE_ABORTED_BY_CALLBACK',
[43] => 'CURLE_BAD_FUNCTION_ARGUMENT',
[45] => 'CURLE_INTERFACE_FAILED',
[47] => 'CURLE_TOO_MANY_REDIRECTS',
[48] => 'CURLE_UNKNOWN_TELNET_OPTION',
[49] => 'CURLE_TELNET_OPTION_SYNTAX',
[51] => 'CURLE_PEER_FAILED_VERIFICATION',
[52] => 'CURLE_GOT_NOTHING',
[53] => 'CURLE_SSL_ENGINE_NOTFOUND',
[54] => 'CURLE_SSL_ENGINE_SETFAILED',
[55] => 'CURLE_SEND_ERROR',
[56] => 'CURLE_RECV_ERROR',
[58] => 'CURLE_SSL_CERTPROBLEM',
[59] => 'CURLE_SSL_CIPHER',
[60] => 'CURLE_SSL_CACERT',
[61] => 'CURLE_BAD_CONTENT_ENCODING',
[62] => 'CURLE_LDAP_INVALID_URL',
[63] => 'CURLE_FILESIZE_EXCEEDED',
[64] => 'CURLE_USE_SSL_FAILED',
[65] => 'CURLE_SEND_FAIL_REWIND',
[66] => 'CURLE_SSL_ENGINE_INITFAILED',
[67] => 'CURLE_LOGIN_DENIED',
[68] => 'CURLE_TFTP_NOTFOUND',
[69] => 'CURLE_TFTP_PERM',
[70] => 'CURLE_REMOTE_DISK_FULL',
[71] => 'CURLE_TFTP_ILLEGAL',
[72] => 'CURLE_TFTP_UNKNOWNID',
[73] => 'CURLE_REMOTE_FILE_EXISTS',
[74] => 'CURLE_TFTP_NOSUCHUSER',
[75] => 'CURLE_CONV_FAILED',
[76] => 'CURLE_CONV_REQD',
[77] => 'CURLE_SSL_CACERT_BADFILE',
[78] => 'CURLE_REMOTE_FILE_NOT_FOUND',
[79] => 'CURLE_SSH',
[80] => 'CURLE_SSL_SHUTDOWN_FAILED',
[81] => 'CURLE_AGAIN',
[82] => 'CURLE_SSL_CRL_BADFILE',
[83] => 'CURLE_SSL_ISSUER_ERROR',
[84] => 'CURLE_FTP_PRET_FAILED',
[84] => 'CURLE_FTP_PRET_FAILED',
[85] => 'CURLE_RTSP_CSEQ_ERROR',
[86] => 'CURLE_RTSP_SESSION_ERROR',
[87] => 'CURLE_FTP_BAD_FILE_LIST',
[88] => 'CURLE_CHUNK_FAILED');

?>

To coder.ua[at]gmail.com:

Please note some errors have more than one constants:

[CURLE_FTP_PARTIAL_FILE]    18
[CURLE_PARTIAL_FILE]    18

[CURLE_HTTP_NOT_FOUND]    22
[CURLE_HTTP_RETURNED_ERROR]    22

[CURLE_OPERATION_TIMEDOUT]    28
[CURLE_OPERATION_TIMEOUTED]    28

[CURLE_BAD_DOWNLOAD_RESUME]    36
[CURLE_FTP_BAD_DOWNLOAD_RESUME]    36

Note that you can detect errors using curl_multi_info_read() in the curl_multi_exec() loop that don't show up later using curl_errno().  This seems particularly true of connection errors.

All available curl error constants and values in PHP 5.4.7 on FreeBSD 8.2-RELEASE:

CURLE_UNSUPPORTED_PROTOCOL => 1
CURLE_FAILED_INIT => 2
CURLE_URL_MALFORMAT => 3
CURLE_URL_MALFORMAT_USER => 4
CURLE_COULDNT_RESOLVE_PROXY => 5
CURLE_COULDNT_RESOLVE_HOST => 6
CURLE_COULDNT_CONNECT => 7
CURLE_FTP_WEIRD_SERVER_REPLY => 8
CURLE_FTP_ACCESS_DENIED => 9
CURLE_FTP_USER_PASSWORD_INCORRECT => 10
CURLE_FTP_WEIRD_PASS_REPLY => 11
CURLE_FTP_WEIRD_USER_REPLY => 12
CURLE_FTP_WEIRD_PASV_REPLY => 13
CURLE_FTP_WEIRD_227_FORMAT => 14
CURLE_FTP_CANT_GET_HOST => 15
CURLE_FTP_CANT_RECONNECT => 16
CURLE_FTP_COULDNT_SET_BINARY => 17
CURLE_PARTIAL_FILE => 18
CURLE_FTP_COULDNT_RETR_FILE => 19
CURLE_FTP_WRITE_ERROR => 20
CURLE_FTP_QUOTE_ERROR => 21
CURLE_HTTP_NOT_FOUND => 22
CURLE_WRITE_ERROR => 23
CURLE_MALFORMAT_USER => 24
CURLE_FTP_COULDNT_STOR_FILE => 25
CURLE_READ_ERROR => 26
CURLE_OUT_OF_MEMORY => 27
CURLE_OPERATION_TIMEOUTED => 28
CURLE_FTP_COULDNT_SET_ASCII => 29
CURLE_FTP_PORT_FAILED => 30
CURLE_FTP_COULDNT_USE_REST => 31
CURLE_FTP_COULDNT_GET_SIZE => 32
CURLE_HTTP_RANGE_ERROR => 33
CURLE_HTTP_POST_ERROR => 34
CURLE_SSL_CONNECT_ERROR => 35
CURLE_FTP_BAD_DOWNLOAD_RESUME => 36
CURLE_FILE_COULDNT_READ_FILE => 37
CURLE_LDAP_CANNOT_BIND => 38
CURLE_LDAP_SEARCH_FAILED => 39
CURLE_LIBRARY_NOT_FOUND => 40
CURLE_FUNCTION_NOT_FOUND => 41
CURLE_ABORTED_BY_CALLBACK => 42
CURLE_BAD_FUNCTION_ARGUMENT => 43
CURLE_BAD_CALLING_ORDER => 44
CURLE_HTTP_PORT_FAILED => 45
CURLE_BAD_PASSWORD_ENTERED => 46
CURLE_TOO_MANY_REDIRECTS => 47
CURLE_UNKNOWN_TELNET_OPTION => 48
CURLE_TELNET_OPTION_SYNTAX => 49
CURLE_OBSOLETE => 50
CURLE_SSL_PEER_CERTIFICATE => 51
CURLE_GOT_NOTHING => 52
CURLE_SSL_ENGINE_NOTFOUND => 53
CURLE_SSL_ENGINE_SETFAILED => 54
CURLE_SEND_ERROR => 55
CURLE_RECV_ERROR => 56
CURLE_SHARE_IN_USE => 57
CURLE_SSL_CERTPROBLEM => 58
CURLE_SSL_CIPHER => 59
CURLE_SSL_CACERT => 60
CURLE_BAD_CONTENT_ENCODING => 61
CURLE_LDAP_INVALID_URL => 62
CURLE_FILESIZE_EXCEEDED => 63
CURLE_FTP_SSL_FAILED] => 64

The manual above only mentions how to get an error description from the cURL handle ($ch) using curl_error():
<?php
$description = curl_error($ch);
?>

But you can also get a description from the error number itself using curl_strerror():
<?php
$description = curl_strerror(curl_errno($ch));
?>

The latter can be useful if you only have access to the error number but not to the cURL handle.

you should look on http://php.net/manual/en/curl.constants.php for the error codes, not on the linked official curl doc. some of the error codes are different, at least the one mentioned on the other page:

in PHP the curl error number 28 is called

CURLE_OPERATION_TIMEOUTED

while in the official site is:

CURLE_OPERATION_TIMEDOUT

CURLcode v7.81 reversed array for PHP  (from \curl\include\curl\curl.h l.495)
https://gist.github.com/nono303/de1256f55e26fdbcc3e0d72e397ac4a1

/!\ ATTENTION if you are using curl multi handles

This function returns 0 in error conditions when run on curl handles accessed via curl_multi_info_read().

You must look at the data returned by curl_multi_info_read() for the true errno. The data is over there instead.

Gets a current list of all CURLE_* error constants, their applicable error messages, and their applicable error codes. Useful for making wrapper classes or for mapping cURL errors to specific Exception objects.

<?php

$list   = get_defined_constants(TRUE);
$list   = preg_grep('/^CURLE_/', array_flip($list['curl']));
$result = array();

foreach ($list as $const) {
    $result[$const] = array
    (
        'code'    => constant($const),
        'message' => curl_strerror(constant($const))
    );
}

echo "<pre>";
echo print_r($result, TRUE);
echo "</pre>";