diff --git a/src/wp-includes/PHPMailer/PHPMailer.php b/src/wp-includes/PHPMailer/PHPMailer.php
index 13420c81dd..0f23b041e2 100644
--- a/src/wp-includes/PHPMailer/PHPMailer.php
+++ b/src/wp-includes/PHPMailer/PHPMailer.php
@@ -750,7 +750,7 @@ class PHPMailer
      *
      * @var string
      */
-    const VERSION = '6.6.2';
+    const VERSION = '6.6.3';
 
     /**
      * Error severity: message only, continue processing.
@@ -1557,17 +1557,17 @@ class PHPMailer
 
             //Validate From, Sender, and ConfirmReadingTo addresses
             foreach (['From', 'Sender', 'ConfirmReadingTo'] as $address_kind) {
-                $this->$address_kind = trim($this->$address_kind);
-                if (empty($this->$address_kind)) {
+                $this->{$address_kind} = trim($this->{$address_kind});
+                if (empty($this->{$address_kind})) {
                     continue;
                 }
-                $this->$address_kind = $this->punyencodeAddress($this->$address_kind);
-                if (!static::validateAddress($this->$address_kind)) {
+                $this->{$address_kind} = $this->punyencodeAddress($this->{$address_kind});
+                if (!static::validateAddress($this->{$address_kind})) {
                     $error_message = sprintf(
                         '%s (%s): %s',
                         $this->lang('invalid_address'),
                         $address_kind,
-                        $this->$address_kind
+                        $this->{$address_kind}
                     );
                     $this->setError($error_message);
                     $this->edebug($error_message);
@@ -1667,7 +1667,7 @@ class PHPMailer
                 default:
                     $sendMethod = $this->Mailer . 'Send';
                     if (method_exists($this, $sendMethod)) {
-                        return $this->$sendMethod($this->MIMEHeader, $this->MIMEBody);
+                        return $this->{$sendMethod}($this->MIMEHeader, $this->MIMEBody);
                     }
 
                     return $this->mailSend($this->MIMEHeader, $this->MIMEBody);
@@ -2202,7 +2202,8 @@ class PHPMailer
         //As we've caught all exceptions, just report whatever the last one was
         if ($this->exceptions && null !== $lastexception) {
             throw $lastexception;
-        } elseif ($this->exceptions) {
+        }
+        if ($this->exceptions) {
             // no exception was thrown, likely $this->smtp->connect() failed
             $message = $this->getSmtpErrorMessage('connect_host');
             throw new Exception($message);
@@ -3715,20 +3716,21 @@ class PHPMailer
      * These differ from 'regular' attachments in that they are intended to be
      * displayed inline with the message, not just attached for download.
      * This is used in HTML messages that embed the images
-     * the HTML refers to using the $cid value.
+     * the HTML refers to using the `$cid` value in `img` tags, for example `<img src="cid:mylogo">`.
      * Never use a user-supplied path to a file!
      *
      * @param string $path        Path to the attachment
      * @param string $cid         Content ID of the attachment; Use this to reference
      *                            the content when using an embedded image in HTML
-     * @param string $name        Overrides the attachment name
-     * @param string $encoding    File encoding (see $Encoding)
-     * @param string $type        File MIME type
-     * @param string $disposition Disposition to use
-     *
-     * @throws Exception
+     * @param string $name        Overrides the attachment filename
+     * @param string $encoding    File encoding (see $Encoding) defaults to `base64`
+     * @param string $type        File MIME type (by default mapped from the `$path` filename's extension)
+     * @param string $disposition Disposition to use: `inline` (default) or `attachment`
+     *                            (unlikely you want this – {@see `addAttachment()`} instead)
      *
      * @return bool True on successfully adding an attachment
+     * @throws Exception
+     *
      */
     public function addEmbeddedImage(
         $path,
@@ -4106,12 +4108,8 @@ class PHPMailer
             //Is it a valid IPv4 address?
             return filter_var($host, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4) !== false;
         }
-        if (filter_var('http://' . $host, FILTER_VALIDATE_URL) !== false) {
-            //Is it a syntactically valid hostname?
-            return true;
-        }
-
-        return false;
+        //Is it a syntactically valid hostname (when embeded in a URL)?
+        return filter_var('http://' . $host, FILTER_VALIDATE_URL) !== false;
     }
 
     /**
@@ -4581,7 +4579,7 @@ class PHPMailer
     public function set($name, $value = '')
     {
         if (property_exists($this, $name)) {
-            $this->$name = $value;
+            $this->{$name} = $value;
 
             return true;
         }
diff --git a/src/wp-includes/PHPMailer/SMTP.php b/src/wp-includes/PHPMailer/SMTP.php
index 64fd078267..c439a888ba 100644
--- a/src/wp-includes/PHPMailer/SMTP.php
+++ b/src/wp-includes/PHPMailer/SMTP.php
@@ -35,7 +35,7 @@ class SMTP
      *
      * @var string
      */
-    const VERSION = '6.6.2';
+    const VERSION = '6.6.3';
 
     /**
      * SMTP line break constant.