updating min PHP version

manual/new/docs/en/getting-started.txt

 ++ Requirements
-Doctrine requires PHP >= 5.1. it doesn't require any external libraries. For database function call abstraction Doctrine uses PDO which is bundled with php by default.
+Doctrine requires PHP >= 5.2. it doesn't require any external libraries. For database function call abstraction Doctrine uses PDO which is bundled with php by default.
 
 ++ Installation
 ++ Compiling