无法通过PHPMailer发送邮件

This is my php code

require("class.phpmailer.php");

$mail = new PHPMailer();

$mail->IsSMTP();  // telling the class to use SMTP
$mail->Host     = "smtp.gmail.com"; // SMTP server

$mail->From     = "example@gmail.com";
$mail->AddAddress("example@yahoo.com");

$mail->Subject  = "First PHPMailer Message";
$mail->Body     = "Hi! 

 This is my first e-mail sent through PHPMailer.";
$mail->WordWrap = 50;

if(!$mail->Send()) {
echo 'Message was not sent.';
echo 'Mailer error: ' . $mail->ErrorInfo;
} else {
echo 'Message has been sent.';
}

My php.ini has this mail configuration [mail function]

SMTP = smtp.gmail.com
smtp_port = 465
sendmail_from =example@gmail.com

My error is

The following From address failed: example@gmail.com Message was not sent.Mailer error: The following From address failed: example@gmail.com SMTP server error: 5.7.0 Must issue a STARTTLS command first. i1sm13250552pbv.49

please help me.thanks in advance......