Add ability to use formatter in email subject lines

src/Monolog/Handler/MailHandler.php

@@ -52,4 +52,16 @@ abstract class MailHandler extends AbstractProcessingHandler
     {
         $this->send((string) $record['formatted'], array($record));
     }
+
+    protected function getHighestRecord(array $records)
+    {
+        $highestRecord = null;
+        foreach ($records as $record) {
+            if ($highestRecord === null || $highestRecord['level'] < $record['level']) {
+                $highestRecord = $record;
+            }
+        }
+
+        return $highestRecord;
+    }
 }

src/Monolog/Handler/NativeMailerHandler.php

@@ -12,6 +12,7 @@
 namespace Monolog\Handler;
 
 use Monolog\Logger;
+use Monolog\Formatter\LineFormatter;
 
 /**
  * NativeMailerHandler uses the mail() function to send the emails
@@ -122,9 +123,16 @@ class NativeMailerHandler extends MailHandler
         if ($this->getContentType() == 'text/html' && false === strpos($headers, 'MIME-Version:')) {
             $headers .= 'MIME-Version: 1.0' . "\r\n";
         }
+
+        $subject = $this->subject;
+        if ($records) {
+            $subjectFormatter = new LineFormatter($this->subject);
+            $subject = $subjectFormatter->format($this->getHighestRecord($records));
+        }
+
         $parameters = implode(' ', $this->parameters);
         foreach ($this->to as $to) {
-            mail($to, $this->subject, $content, $headers, $parameters);
+            mail($to, $subject, $content, $headers, $parameters);
         }
     }
 

src/Monolog/Handler/SwiftMailerHandler.php

@@ -12,6 +12,7 @@
 namespace Monolog\Handler;
 
 use Monolog\Logger;
+use Monolog\Formatter\LineFormatter;
 
 /**
  * SwiftMailerHandler uses Swift_Mailer to send the emails
@@ -66,6 +67,11 @@ class SwiftMailerHandler extends MailHandler
             throw new \InvalidArgumentException('Could not resolve message as instance of Swift_Message or a callable returning it');
         }
 
+        if ($records) {
+            $subjectFormatter = new LineFormatter($message->getSubject());
+            $message->setSubject($subjectFormatter->format($this->getHighestRecord($records)));
+        }
+
         $message->setBody($content);
         $message->setDate(time());