Description:
------------
The current state of string slicing in PHP is very confusing and non-obvious.

The substr() function attempted some nice string slicing features, but it just 
isn't a good interface for string slicing and due to legacy concerns it cannot 
be changed (I assume).

substr() problems:
- The interface is inconsistent and confusing.  When length is positive, then 
the function effectively returns from [start, start+length).  When length is 
negative, the function returns [start, str_len+length).  str_slice() has a much 
simpler interface: It returns from [start, end), and start and end may be 
defined as positive numbers from the start of the string or negative numbers 
from the end of the string.

- Returning FALSE when start + length parameters are invalid.  This is annoying 
because when using this function you always have to deal with this FALSE case if 
you need a string.  Here is an example where str_slice() produces a more logical 
output than substr:

print var_dump(substr('', -1) == '0'); // TRUE
print var_dump(str_slice('', -1) == '0'); // FALSE

This patch effectively solves the following bug as well:
http://bugs.php.net/bug.php?id=38437 (substr should return '' instead of FALSE)
http://bugs.php.net/bug.php?id=50434 (add str_startswith and str_endswith 
functions, which are trivial with str_slice, even in corner cases)

Test script:
---------------
Test is attached in the patch.