add the isSMTPConnected() method for a type-safe check

Author: michalsn

system/Email/Email.php

@@ -1886,7 +1886,7 @@ protected function SMTPEnd()
      */
     protected function SMTPConnect()
     {
-        if (is_resource($this->SMTPConnect)) {
+        if ($this->isSMTPConnected()) {
             return true;
         }
 
@@ -1910,7 +1910,7 @@ protected function SMTPConnect()
             $this->SMTPTimeout,
         );
 
-        if (! is_resource($this->SMTPConnect)) {
+        if (! $this->isSMTPConnected()) {
             $this->setErrorMessage(lang('Email.SMTPError', [$errno . ' ' . $errstr]));
 
             return false;
@@ -2227,7 +2227,7 @@ protected function mimeTypes($ext = '')
 
     public function __destruct()
     {
-        if (is_resource($this->SMTPConnect)) {
+        if ($this->isSMTPConnected()) {
             try {
                 $this->sendCommand('quit');
             } catch (ErrorException $e) {
@@ -2284,4 +2284,14 @@ protected function setArchiveValues(): array
 
         return $this->archive;
     }
+
+    /**
+     * Checks if there is an active SMTP connection.
+     *
+     * @return bool True if SMTP connection is established and open, false otherwise
+     */
+    protected function isSMTPConnected(): bool
+    {
+        return $this->SMTPConnect !== null && get_debug_type($this->SMTPConnect) !== 'resource (closed)';
+    }
 }