diff --git a/src/Symfony/Component/Process/Process.php b/src/Symfony/Component/Process/Process.php
index bd0fb18572..3364e6c2b7 100644
--- a/src/Symfony/Component/Process/Process.php
+++ b/src/Symfony/Component/Process/Process.php
@@ -21,6 +21,9 @@ namespace Symfony\Component\Process;
  */
 class Process
 {
+    const ERR = 'err';
+    const OUT = 'out';
+
     private $commandline;
     private $cwd;
     private $env;
@@ -93,7 +96,7 @@ class Process
         $that = $this;
         $callback = function ($type, $data) use ($that, $callback)
         {
-            if ('out' == $type) {
+            if (self::OUT == $type) {
                 $that->addOutput($data);
             } else {
                 $that->addErrorOutput($data);
@@ -156,7 +159,7 @@ class Process
                 $type = array_search($pipe, $pipes);
                 $data = fread($pipe, 8192);
                 if (strlen($data) > 0) {
-                    call_user_func($callback, $type == 1 ? 'out' : 'err', $data);
+                    call_user_func($callback, $type == 1 ? self::OUT : self::ERR, $data);
                 }
                 if (false === $data || feof($pipe)) {
                     fclose($pipe);