That's because normal string escape rules still apply, and \t is the tab character.

I whole-heartedly disagree that this is not a bug, as does the PHP.net 
documentation itself!  If you look at the documentation, "\t" is used all over 
in the examples to represent how to type the letter t. For example, these are 
copied from the documentation:

$today = date('\i\t \i\s \t\h\e jS \d\a\y.');
$today = date('H:m:s \m \i\s\ \m\o\n\t\h');

The backslash in the tab character should have to be escaped, so the tab 
character should be \\\t.  What's worse is this is not expected behavior and 
there is no solution or note posted in the php documentation, leaving many 
people confused.

dillydadally I think you missed the fact that the original report had double-
quoted strings. Your version with single-quoted strings works as you would 
expect.
eg.

php > echo date('H:m:s \m \i\s\ \m\o\n\t\h');
16:05:40 m is month

Ah, you're right!  The distinction completely slipped my mind!  

I think the documentation is commenting on this here:

"You can prevent a recognized character in the format string from being expanded 
by escaping it with a preceding backslash. If the character with a backslash is 
already a special sequence, you may need to also escape the backslash."

I took this to mean the backslash in a tab character would have to be escaped, 
but on a second review, I understand what it's saying.  It's a bit confusing.  
Perhaps an example in the documentation would help clear things up.  Or maybe 
I'm just wrong.  Anyway, if anyone hits this and wants to use double quotes, you 
can still do a t by typing \\t.