Description:
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The short open tag is very useful for readable templates: <?=$var?>, <? foreach ($array as $v): ?>, <? foreachend; ?> etc.

But on the other hand it interferes with other processing instructions like
<?xml version="1.0" encoding="utf-8"?> or <?xml-stylesheet href="style.css" type="text/css" media="all" ?>

So you have to use
<?="<?"?>xml version="1.0" encoding="utf-8"?>
or something similar in your template which makes it less readable.

The compiler should distinguish between a short PHP tag and other processing instructions. If the compiler is not sure if "<? something" is PHP code or not it should report an error as before.

Test script:
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<?xml version="1.0" encoding="utf-8"?>
<?="hello world\n";?>
<?php echo "hello world\n"; ?>


Expected result:
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<?xml version="1.0" encoding="utf-8"?>
hello world
hello world


Actual result:
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Parse error: syntax error, unexpected 'version' (T_STRING) in ... on line 1