(bug-fix:minor) Avoid 'undefined index' warnings by making sure to access to $_POST and $_GET keys only if they exist.
[php] (bug-fix:minor) Solved 'use of undefined constant id' error.
[php] (bug-fix:minor) Solved 'undefined index: childs' error.
[php] (bug-fix:major) Solved 'undefined variable: filter' and 'undefined variable: display' errors (did not get the relative parameter values).
[php] (bug-fix:minor) Solved 'undefined index: filter' error.
[php] (change:minor) Replaced '== null'/'\!= null' with the 'is_null' function
[php] (bug-fix:minor) Solved 'Undefined variable: table' error.
[php] (new:minor) Improved error messages and also redirect them to the server log.
[php] (change) Added methods 'getSessionPostGetParam'/'getSessionGetPostParam' as helpers for getting session parameters
[php] (bug-fix) Wrong variable name introduced in the last commit: 'category' => 'categoryId'
[php] (bug-fix) Wrong session parameter name introduced in the last commit: 'categoryId' => 'category'
[php] (bug-fix) HTML document type definition was not conformant to W3C standard
[php] (change) Cosmetic changes: style for error messages confined in the 'printError' function
Merge branch 'master' of https://github.com/sguazt/RUBBoS into php-dev