['foo' => 'bar'] array notation is not PHP 5.3 compatible.

src/Monolog/Handler/SyslogUdp/UdpSocket.php

@@ -52,7 +52,7 @@ class UdpSocket
             $remaining = null;
         }
 
-        return [$chunk, $remaining];
+        return array($chunk, $remaining);
     }
 
     protected function shouldSplitLine($remaining, $header)

src/Monolog/Handler/SyslogUdpHandler.php

@@ -71,7 +71,7 @@ class SyslogUdpHandler extends AbstractProcessingHandler
 
     protected function write(array $record)
     {
-        $this->_write(['message' => $record['formatted'], 'priority' => $record['level']]);
+        $this->_write(array('message' => $record['formatted'], 'priority' => $record['level']));
     }
 
     protected function _write($event)