PHP does not seem to recognize the "\v" escape sequence within strings, like C does.  This would probably be an issue in the PHP double-quoted string parser.

I'm guessing this has been filed elsewhere as a bug, but I cannot find it (the bug search form does not like \v, '\v' or "\v").

Even though this is documented (language.types.string.php), it is still unexpected.  Consider the following code:

$s = ereg_replace("[\t\v\n\r ]", '', $s);

The intent is to replace all whitespace, where whitespace is defined similarly to isspace(3) (but I forgot '\f', no big deal).  The result is that whitespace *and* the "v" character is removed.  The letter v is rare enough in English (and vertical tab is almost unheard-of in text) that this can go for a few days without being noticed.

In the same vein, \f should also be recognized.  I'm not sure about \a and \b, but I would definitely say trigraphs are a bad idea :).

Perhaps only a warning would be enough.  My copy of the C standard states that when an unknown escape sequence is encountered, the compiler should emit a diagnostic.  That seems like reasonable behaviour.