PHP: Add support for nested arrays

Fixes bug 2240
PHP: Add support for blank spaces in array declaration

Fixes bug #2646
PHP: Update module docstring
Add PHP storage test to check whitespace in nested arrays declaration
PHP: Add code to warn about duplicate entries

Such entries throw the error "'NoneType' object has no attribute 'getnotes'"
when converting to po using a template, which is hard to trace, so better
warn and abort execution.