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<? is used in XML headers.

OTOH, you shouldn't ever be generating XML headers from PHP anyway (XHTML is a dead-end, and raw XML should be generated by an XML library), so this shouldn't be that big of an issue.

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But, less than question mark and a space are not a valid XML combination. XML processing instructions cannot begin with a space. Likewise, PHP cannot run commands into the short tags, like <?echo $foo; ?>. So, there really is no collision, it's just grandstanding.

I wonder if the XML PI spec says anything about <?= Can you have a PI start with = ? If not, the echo short tag should stay for certain.

'<?=' is invalid XML. '<?' must be followed by a valid XML Name.

See http://www.w3.org/TR/2008/REC-xml-20081126/#sec-pi

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