Why Does PHPMailer Emit a PHP Warning About a Mismatched Certificate in PHP 5.6?

PHPMailer Emits PHP Warning Due to Mismatched Certificate

PHPMailer users may encounter a warning when connecting to SMTP hosts on PHP 5.6:

PHP Warning: stream_socket_enable_crypto(): Peer certificate did not match expected

This warning stems from the increased security in PHP 5.6, which strictly verifies SSL certificates. The warning suggests that the certificate presented by the SMTP host does not match the expected certificate for that host.

Root Cause:

The mismatch occurs when the SMTP host presents a certificate that does not have a common name (CN) that matches the expected hostname. For example, if you are connecting to mx1.sub4.homie.mail.dreamhost.com and the certificate CN is *.mail.dreamhost.com, PHP will raise the warning.

Solution:

To resolve this issue, there are two approaches:

1. Contact the SMTP Host: Request that the host update its SSL certificate to have a CN that matches the hostname you are connecting to. This is the recommended and secure solution.
2. Configure PHPMailer to Ignore Certificate Verification: You can configure PHPMailer to ignore the certificate mismatch by setting the following SMTPOptions:

$mail->SMTPOptions = array(
    'ssl' => array(
        'verify_peer'  => false,
        'verify_peer_name'  => false,
        'allow_self_signed' => true
    )
);

Note: Disabling certificate verification is not recommended and should be used only as a temporary solution.

Conclusion:

The PHP Warning "stream_socket_enable_crypto(): Peer certificate did not match expected" can be addressed by either contacting the SMTP host to update their certificate or by configuring PHPMailer to ignore certificate verification. The recommended solution is to have the SMTP host provide a valid certificate that matches the hostname.

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