Description:
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I expect that
print "\a";
will fail because there is no entity like \a among
\n, \r, \t, \\, \$, \", \[0-7]{1,3} and \x[0-9A-Fa-f]{1,2}.

It is like that browsers like IE shows "&abc" as there were written "&abc".

It's harmful.

More than that, I and some other PHP experts have been puzzleing why this string works:

$tokens   = preg_split('/(?<!\\\)[&?!]/', $query, -1,
                               PREG_SPLIT_DELIM_CAPTURE);

Everybody expected that ) was escaped!

Reproduce code:
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$tokens   = preg_split('/(?<!\\\)[&?!]/', $query, -1,
                               PREG_SPLIT_DELIM_CAPTURE);

Expected result:
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PHP fails because of brackets mismatch (one of brackets is escaped).

Actual result:
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It works :(