Expected but undocumented behavior.

Single characters correspond to military timezones. See
http://en.wikipedia.org/wiki/List_of_military_time_zones

foreach (range("a", "z") as $l) {
    if ($l == "j") continue;
    printf("%s = %s\n", $l, (new DateTime($l))->format("r"));
}

So the list of timezones documentation should be updated to mention these.

IMO, the one that should be fixed is strtotime documentation,
since this behaviour came from thus function. As doumentation say,
the first argument should be time string, that -i think- will be
parsed by strtotime function.

Test script:
---------------
echo date('Y-m-d H:i:s',strtotime('now'))."\n";
echo date('Y-m-d H:i:s',strtotime('a'))."\n";

Actual result:
--------------
2013-09-30 10:07:18
2013-09-30 16:07:18

Is the bug in the constructor of DateTimeZone then? This does not recognize the 
single letter timezones.

$tz = new \DateTimeZone('z');

Will raise an exception.

It is not a bug, but a feature that not documented.
As requinix says, single character could be mean
military time zone. for example Z1500 mean 15 o'clock
current date at GMT.

Test script:
---------------
echo date('Y-m-d H:i:s', strtotime('Z1500'));

Actual result:
--------------
2013-09-30 22:00:00

my time zone is +7

please try to take a look at this documentation

http://php.net/datetime.construct
http://php.net/datetime.formats
http://php.net/datetime.formats.time

btw, thus single char timezone is already documented.
take a look at tz format.. it declared as [A-Za-z]{1,6}

That doesn't explain why DateTimeZone::construct() doesn't recognise a valid time zone like 'z', nor does it explain why

$dateTime = new \DateTime(null, 'z');

raises an exception. If 'z' is a valid tz, it should not.

There is a bug in there somewhere.

For reference, requinix' script from above with its output:
<https://3v4l.org/1sqnT>.

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.

I've learned that those military timezones are only a small subset
of allowed timezone abbreviations (see
timezone_abbreviations_list()). These are all documented now.

It has been decided that it's better not to document the timezone
abbreviations due to their volatility.