i want to use reply a mail giving senders sending date as reference.

Asking for the sender's Date header is common. @vipindas wanted the sending date placed into a reply and @Puckdropper suggested regex. A reliable flow is: obtain the raw headers (IMAP/POP or piped mail), "unfold" header lines, locate the Date header (fall back to the last Received line if Date is missing), parse it with PHP's date facilities, then format the result for the reply. Regex alone can work, but use DateTime or strtotime to avoid brittle string handling.

Example: extract from raw headers, unfold and parse.

$raw = $rawHeaders;                      // raw headers string
$raw = preg_replace("/\r?\n\s+/", " ", $raw); // unfold folded headers

if (preg_match('/^Date:\s*(.+)$/mi', $raw, $m)) {
    $dateStr = trim($m[1]);
} elseif (preg_match_all('/^Received:.*;\s*(.+)$/m', $raw, $mm)) {
    $dateStr = trim(end($mm[1])); // use last Received date
} else {
    $dateStr = null;
}

if ($dateStr) {
    try {
        $dt = new DateTime($dateStr);
    } catch (Exception $e) {
        $ts = strtotime($dateStr);
        $dt = $ts ? new DateTime("@$ts") : null;
    }
    if ($dt) {
        $dt->setTimezone(new DateTimeZone('UTC')); // normalize as needed
        $replyDate = $dt->format('d/m/Y');
        $body = "ref: this is in response to your mail dated $replyDate ...";
    }
}

If messages are read from a mailbox, the IMAP helper returns a parsed date: imap_headerinfo($stream,$msgno)->date. See the extension docs for details. Be aware that Date headers can be malformed or forged; treat the value as a display field, not a trusted timestamp. For parsing and formatting, consult the PHP DateTime and strtotime references: DateTime class, strtotime, imap_headerinfo.