连接gmail通过PHP imap?本地颁发者证书错误

我需要使用PHP通过IMAP连接到Gmail帐户。我有一些工作代码，但是当尝试建立连接时，我收到以下错误:

警告:imap_open() [function。无法打开流{imap.gmail.com:993/ssl}[Gmail]/所有邮件在/home/demoosize/public_html/goqule/email.php在第31行
无法连接到Gmail: imap.gmail.com证书失败:无法获得本地发行者证书:/C=US/O=Google Inc/CN=Google Internet Authority

我的代码在

<?php
    /* connect to gmail */
    $hostname = '{imap.gmail.com:993/ssl}[Gmail]/All Mail';
    $username = 'hidden';
    $password = 'hidden';
    /* try to connect */
    $inbox = imap_open($hostname,$username,$password) or die('Cannot connect to Gmail: ' . imap_last_error());
    /* grab emails */
    $emails = imap_search($inbox,'ALL');
    /* if emails are returned, cycle through each... */
    if($emails) {
      /* begin output var */
      $output = '';
      /* put the newest emails on top */
      rsort($emails);
      /* for every email... */
      foreach($emails as $email_number) {
        /* get information specific to this email */
        $overview = imap_fetch_overview($inbox,$email_number,0);
        $message = imap_fetchbody($inbox,$email_number,2);
        /* output the email header information */
        $output.= '<div class="toggler '.($overview[0]->seen ? 'read' : 'unread').'">';
        $output.= '<span class="subject">'.$overview[0]->subject.'</span> ';
        $output.= '<span class="from">'.$overview[0]->from.'</span>';
        $output.= '<span class="date">on '.$overview[0]->date.'</span>';
        $output.= '</div>';
        /* output the email body */
        $output.= '<div class="body">'.$message.'</div>';
      }
      echo $output;
    } 
    /* close the connection */
    imap_close($inbox);
    ?>