it already does report an error. something like this:
"Notice: Undefined variable: a in C:\wamp\www\testcenter\index.PHP on line 40"
maybe you didn't go specific enough. but you should try error_reporting(-1); as as if enforces the php to show some recomendations. a piece from the php manual about E_STRICT errors:
Enable to have PHP suggest changes to your code which will ensure the best interoperability and forward compatibility of your code.
just remember that error_reporting(-1); shows more errors than error_reporting(E_ALL); because E_STRICT errors are not included in the E_ALL constraint.