Fix PHP error "Only variables should be passed by reference"

--HG--
branch : develop

code/web/public_php/tools/nel_message.php

@@ -86,9 +86,10 @@
 			}
 			else
 			{
-				$this->serialUInt32(strlen($val));
+				$valLen = strlen($val);
+				$this->serialUInt32($valLen);
 				$this->Buffer .= $val;
-				$this->Pos += strlen($val);
+				$this->Pos += $valLen;
 				debug(sprintf ("write string '%s' %d<br>\n", $val, $this->Pos));
 			}
 		}